test(spreadsheet): cover remaining data.rs dependency-graph branches

Targets the uncovered branches the coverage report named, around the
formula dependency graph and incremental recalculation:

- Formula replacement/removal edge cases: set_cell("") removes the
  cell and its edges, formula->value and formula->formula rewiring
  drop stale dependency edges.
- No-op removal paths: set_cell("") on a missing cell records nothing.
- Dependency-graph cleanup after formula deletion: remove_cell on a
  formula cell, and update_dependency_graph's defensive path when a
  dependent has no dependents entry.
- Affected-cell recalculation error paths: non-formula cells inside a
  cycle-affected set keep their raw value; the ="" empty-result
  regression; the recursive eval slow path (cached AST, parse
  fallback, and CycleDetected); parse_cell_computed_value arms; parse
  errors propagating through a dependent and through a large range.

Also covers named-range unary expansion (=-Total), the >64-cell range
fast path, non-numeric number-format fallbacks, and the demo_q3 /
demo_roi constructors.

Engine line coverage: data.rs 91.17% -> 97.90%; engine total
90.09% -> 93.24%. Unit tests 260 -> 279 (19 new); 9 integration tests
unchanged.

crates/apps/spreadsheet/spreadsheet-engine/src/data.rs

@@ -1808,25 +1808,26 @@ impl SpreadsheetData {
                     }
                 }
 
-                // Defensive invariant check (review bug B17):
+                // Defensive invariant check (review bug B17): every cell
-                // every cell that was in `changed_cells` and has a
+                // that was in `changed_cells` and has a formula MUST have
-                // formula MUST now have a non-empty `computed_value`.
+                // been visited by the topological sort. If one was skipped,
-                // If any is empty, it means `topological_sort` failed
+                // the dep-graph walk dropped a cell — a bug. (We cannot use
-                // to visit that cell — a bug in the dep-graph walk.
+                // "computed_value is empty" as the signal: a formula may
+                // legitimately evaluate to the empty string, e.g. `=""`.)
                 // In debug builds this panics so the bug is caught
-                // immediately; in release builds we patch the cell
+                // immediately; in release builds we patch the cell with
-                // with `#ERROR!` so it displays something rather
+                // `#ERROR!` so it displays something actionable.
-                // than appearing empty.
                 //
-                // Note: this check is O(changed_cells.len()), which
+                // Note: this check is O(changed_cells.len()), which is
-                // is typically 1 (single-cell edit) or the size of
+                // typically 1 (single-cell edit) or the size of a
-                // a paste/autofill range. Not a hot path.
+                // paste/autofill range. Not a hot path.
+                let visited: HashSet<CellId> = sorted.iter().copied().collect();
                 for &cell_id in changed_cells {
                     if let Some(cell) = self.cells.get(&cell_id) {
-                        if cell.formula.is_some() && cell.computed_value.is_empty() {
+                        if cell.formula.is_some() && !visited.contains(&cell_id) {
                             debug_assert!(
                                 false,
-                                "recalculate_incremental left cell ({}, {}) with empty computed_value",
+                                "recalculate_incremental did not visit formula cell ({}, {})",
                                 cell_id.row(),
                                 cell_id.col()
                             );
@@ -3024,3 +3025,428 @@ mod formula2_integration_tests {
         assert_eq!(sheet.get_display_value(0, 1), "42");
     }
 }
+
+#[cfg(test)]
+mod dep_graph_branch_tests {
+    use super::*;
+
+    /// Build a plain non-formula cell with a raw value and number format,
+    /// mirroring `cell_with` in `display_value_tests`.
+    fn cell_with(value: &str, fmt: NumberFormat) -> CellData {
+        CellData {
+            value: value.to_string(),
+            style: CellStyle {
+                number_format: fmt,
+                ..Default::default()
+            },
+            ..Default::default()
+        }
+    }
+
+    // ── Formula replacement / removal edge cases ────────────────────────
+
+    /// `set_cell("")` on an existing *formula* cell removes the cell,
+    /// clears its dep-graph edges, recalcs dependents, and records the
+    /// change.
+    #[test]
+    fn set_cell_empty_removes_existing_formula_cell() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "10");
+        sheet.set_cell(0, 1, "=A1");
+        assert_eq!(sheet.get_display_value(0, 1), "10");
+
+        sheet.set_cell(0, 1, "");
+        assert!(!sheet.cells.contains_key(&CellId::new(0, 1)));
+        assert!(!sheet.dependencies.contains_key(&CellId::new(0, 1)));
+        let a1_dependents = sheet.dependents.get(&CellId::new(0, 0));
+        assert!(!a1_dependents.is_some_and(|s| s.contains(&CellId::new(0, 1))));
+        assert_eq!(sheet.get_display_value(0, 1), "");
+    }
+
+    /// Replacing a formula with a constant drops the old dependency edges.
+    #[test]
+    fn set_cell_replacing_formula_with_value_cleans_graph() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "10");
+        sheet.set_cell(0, 1, "=A1");
+        assert!(sheet.dependencies.contains_key(&CellId::new(0, 1)));
+
+        sheet.set_cell(0, 1, "7");
+        assert_eq!(sheet.get_display_value(0, 1), "7");
+        assert!(!sheet.dependencies.contains_key(&CellId::new(0, 1)));
+        assert!(!sheet
+            .dependents
+            .get(&CellId::new(0, 0))
+            .is_some_and(|s| s.contains(&CellId::new(0, 1))));
+    }
+
+    /// Replacing one formula with another re-wires the graph end-to-end
+    /// (through `set_cell`, not `update_dependency_graph` directly).
+    #[test]
+    fn set_cell_replacing_formula_with_formula_rewires_graph() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "1");
+        sheet.set_cell(0, 1, "2");
+        sheet.set_cell(0, 2, "=A1");
+        assert_eq!(sheet.get_display_value(0, 2), "1");
+
+        sheet.set_cell(0, 2, "=B1");
+        assert_eq!(sheet.get_display_value(0, 2), "2");
+        let deps = sheet.dependencies.get(&CellId::new(0, 2)).unwrap();
+        assert!(!deps.contains(&CellId::new(0, 0)));
+        assert!(deps.contains(&CellId::new(0, 1)));
+        assert!(!sheet
+            .dependents
+            .get(&CellId::new(0, 0))
+            .is_some_and(|s| s.contains(&CellId::new(0, 2))));
+        assert!(sheet
+            .dependents
+            .get(&CellId::new(0, 1))
+            .is_some_and(|s| s.contains(&CellId::new(0, 2))));
+    }
+
+    // ── No-op removal paths ─────────────────────────────────────────────
+
+    /// `set_cell("")` on a cell that does not exist is a no-op: no cell,
+    /// no dep-graph entry, no panic.
+    #[test]
+    fn set_cell_empty_on_missing_cell_is_noop() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "");
+        assert!(sheet.cells.is_empty());
+        assert!(sheet.dependencies.is_empty());
+        assert!(sheet.dependents.is_empty());
+    }
+
+    // ── Dependency-graph cleanup after formula deletion ─────────────────
+
+    /// `remove_cell` on a *formula* cell removes its dependency edges and
+    /// the reverse dependent edges of the cells it referenced.
+    #[test]
+    fn remove_cell_formula_cleans_dependency_graph() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "10");
+        sheet.set_cell(0, 1, "=A1");
+        assert!(sheet.dependencies.contains_key(&CellId::new(0, 1)));
+
+        sheet.remove_cell(0, 1);
+        assert!(!sheet.cells.contains_key(&CellId::new(0, 1)));
+        assert!(!sheet.dependencies.contains_key(&CellId::new(0, 1)));
+        assert!(!sheet
+            .dependents
+            .get(&CellId::new(0, 0))
+            .is_some_and(|s| s.contains(&CellId::new(0, 1))));
+    }
+
+    /// `update_dependency_graph`'s defensive branch: an edge listed in
+    /// `dependencies` whose dependent has no `dependents` entry must be
+    /// dropped without panicking.
+    #[test]
+    fn update_dependency_graph_ignores_missing_dependent_entry() {
+        let mut sheet = SpreadsheetData::default();
+        let cell_id = CellId::new(0, 0);
+        let mut deps = HashSet::new();
+        deps.insert(CellId::new(1, 0));
+        // Inconsistent graph: dependencies claims B1, but dependents has
+        // no B1 entry.
+        sheet.dependencies.insert(cell_id, deps);
+        sheet.update_dependency_graph(cell_id, None);
+        assert!(!sheet.dependencies.contains_key(&cell_id));
+    }
+
+    // ── Affected-cell recalculation error paths ─────────────────────────
+
+    /// A formula that evaluates to the empty string must not trip the B17
+    /// "empty computed_value" invariant — that was a false positive on a
+    /// legitimate result (regression for the `=""` panic).
+    #[test]
+    fn empty_text_formula_does_not_panic() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "=\"\"");
+        // Empty computed_value falls back to the raw formula text, as
+        // documented by `display_formula_empty_computed_falls_back_to_value`.
+        assert_eq!(sheet.get_display_value(0, 0), "=\"\"");
+        // A full recalc must also leave it alone rather than panic.
+        sheet.recalculate_all();
+        assert_eq!(sheet.get_display_value(0, 0), "=\"\"");
+    }
+
+    /// When a cycle is detected, non-formula cells in the affected set get
+    /// their computed_value cleared (they keep their raw value), while
+    /// formula cells are marked `#CYCLE!`.
+    #[test]
+    fn cycle_recalc_clears_non_formula_affected_cells() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "1"); // A1 value
+        sheet.set_cell(0, 1, "=A1+C1"); // B1 depends on A1 and C1
+        sheet.set_cell(0, 2, "=B1"); // C1 depends on B1 -> B1<->C1 cycle
+        assert_eq!(sheet.get_display_value(0, 1), "#CYCLE!");
+        assert_eq!(sheet.get_display_value(0, 2), "#CYCLE!");
+
+        // Editing A1 pulls A1, B1 and C1 into the affected set; the cycle
+        // is still there, so B1/C1 stay #CYCLE! while A1 (a value) keeps
+        // its raw value with a cleared computed_value.
+        sheet.set_cell(0, 0, "2");
+        assert_eq!(sheet.get_display_value(0, 0), "2");
+        assert_eq!(sheet.get_display_value(0, 1), "#CYCLE!");
+        assert_eq!(sheet.get_display_value(0, 2), "#CYCLE!");
+    }
+
+    /// Legacy public `get_cell_value` API: missing cells read as 0.0,
+    /// values coerce to Number/Boolean/Text, formulas evaluate, and a
+    /// re-entrant visit reports CycleDetected.
+    #[test]
+    fn public_get_cell_value_covers_all_arms() {
+        let mut sheet = SpreadsheetData::default();
+        let mut visiting = HashSet::new();
+
+        // Missing cell.
+        assert!(matches!(
+            sheet.get_cell_value(0, 0, &mut visiting),
+            Ok(Value::Number(n)) if n == 0.0
+        ));
+
+        // Value cells.
+        sheet.set_cell(0, 0, "42");
+        sheet.set_cell(0, 1, "TRUE");
+        sheet.set_cell(0, 2, "FALSE");
+        sheet.set_cell(0, 3, "hello");
+        assert!(matches!(
+            sheet.get_cell_value(0, 0, &mut visiting),
+            Ok(Value::Number(n)) if n == 42.0
+        ));
+        assert!(matches!(
+            sheet.get_cell_value(0, 1, &mut visiting),
+            Ok(Value::Boolean(b)) if b
+        ));
+        assert!(matches!(
+            sheet.get_cell_value(0, 2, &mut visiting),
+            Ok(Value::Boolean(b)) if !b
+        ));
+        assert!(matches!(
+            sheet.get_cell_value(0, 3, &mut visiting),
+            Ok(Value::Text(t)) if t == "hello"
+        ));
+
+        // Formula evaluation through the recursive path.
+        sheet.set_cell(1, 0, "=A1+1");
+        assert!(matches!(
+            sheet.get_cell_value(1, 0, &mut visiting),
+            Ok(Value::Number(n)) if n == 43.0
+        ));
+
+        // Re-entrant visit -> CycleDetected.
+        let mut visiting = HashSet::new();
+        visiting.insert((1, 0));
+        assert!(matches!(
+            sheet.get_cell_value(1, 0, &mut visiting),
+            Err(FormulaError::CycleDetected)
+        ));
+
+        // parse_cell_ref.
+        assert_eq!(SpreadsheetData::parse_cell_ref("B2"), Some((1, 1)));
+        assert_eq!(SpreadsheetData::parse_cell_ref("not a ref"), None);
+    }
+
+    /// Recursive eval slow path inside `DataEvalContext::get_cell_value`:
+    /// a formula whose computed_value is empty is re-evaluated — cached
+    /// AST first, then the parse fallback — and a self-reference reports
+    /// CycleDetected from the context's own visiting set.
+    #[test]
+    fn eval_context_slow_path_cached_and_parse_fallback() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "42"); // A1
+        sheet.set_cell(0, 1, "=A1+1"); // B1 -> 43, cached AST set
+        sheet.set_cell(0, 2, "=B1+1"); // C1 -> 44
+
+        // Force B1 through the slow path with a cached AST.
+        sheet
+            .cells
+            .get_mut(&CellId::new(0, 1))
+            .unwrap()
+            .computed_value
+            .clear();
+        let mut visiting = HashSet::new();
+        assert!(matches!(
+            sheet.get_cell_value(0, 2, &mut visiting),
+            Ok(Value::Number(n)) if n == 44.0
+        ));
+
+        // Force B1 through the parse-fallback path (no cached AST).
+        let b1 = sheet.cells.get_mut(&CellId::new(0, 1)).unwrap();
+        b1.computed_value.clear();
+        b1.cached_ast = None;
+        let mut visiting = HashSet::new();
+        assert!(matches!(
+            sheet.get_cell_value(0, 2, &mut visiting),
+            Ok(Value::Number(n)) if n == 44.0
+        ));
+
+        // Self-reference with an empty computed value: the recursion
+        // re-enters the cell and the context's visiting set reports the
+        // cycle.
+        sheet.set_cell(2, 0, "=A3");
+        sheet
+            .cells
+            .get_mut(&CellId::new(2, 0))
+            .unwrap()
+            .computed_value
+            .clear();
+        let mut visiting = HashSet::new();
+        assert!(matches!(
+            sheet.get_cell_value(2, 0, &mut visiting),
+            Err(FormulaError::CycleDetected)
+        ));
+    }
+
+    /// `parse_cell_computed_value`: empty -> 0.0, booleans, text, and
+    /// error-string propagation.
+    #[test]
+    fn parse_cell_computed_value_arms() {
+        assert!(matches!(
+            DataEvalContext::parse_cell_computed_value(""),
+            Ok(Value::Number(n)) if n == 0.0
+        ));
+        assert!(matches!(
+            DataEvalContext::parse_cell_computed_value("TRUE"),
+            Ok(Value::Boolean(b)) if b
+        ));
+        assert!(matches!(
+            DataEvalContext::parse_cell_computed_value("false"),
+            Ok(Value::Boolean(b)) if !b
+        ));
+        assert!(matches!(
+            DataEvalContext::parse_cell_computed_value("hello"),
+            Ok(Value::Text(t)) if t == "hello"
+        ));
+        assert!(matches!(
+            DataEvalContext::parse_cell_computed_value("#DIV/0!"),
+            Err(FormulaError::DivByZero)
+        ));
+    }
+
+    // ── Named-range dependency expansion ────────────────────────────────
+
+    /// A unary expression over a named range (`=-Total`) still expands the
+    /// named range into its constituent cells in the dependency graph.
+    #[test]
+    fn unary_named_range_dependencies_expand() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "10");
+        sheet.set_cell(1, 0, "20");
+        sheet.named_ranges.insert("Total".to_string(), (0, 0, 1, 0));
+        sheet.set_cell(2, 0, "=-Total");
+        let deps = sheet.dependencies.get(&CellId::new(2, 0)).unwrap();
+        assert!(deps.contains(&CellId::new(0, 0)));
+        assert!(deps.contains(&CellId::new(1, 0)));
+    }
+
+    // ── Large-range fast path (get_range_values, >64 cells) ─────────────
+
+    /// SUM over a range wider than 64 cells exercises the large-range fast
+    /// path, converting numbers, booleans, text and formula cells.
+    #[test]
+    fn large_range_sum_uses_fast_path() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "10"); // number
+        sheet.set_cell(0, 1, "TRUE"); // boolean (counts as 1 in SUM)
+        sheet.set_cell(0, 2, "FALSE"); // boolean (counts as 0)
+        sheet.set_cell(0, 3, "hello"); // text (skipped by SUM)
+        sheet.set_cell(0, 4, "=A1+5"); // formula with cached value 15
+        sheet.set_cell(8, 8, "7"); // bottom-right number
+        sheet.set_cell(9, 0, "=SUM(A1:I9)");
+        assert_eq!(sheet.get_display_value(9, 0), "33"); // 10+1+0+15+7
+    }
+
+    /// A formula cell inside a large range that was inserted without a
+    /// recalc (`put_cell`) forces the recursive slow path during range
+    /// evaluation, including the parse fallback.
+    #[test]
+    fn large_range_recursive_formula_cell() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "10");
+        let cell = CellData {
+            value: "=A1+1".to_string(),
+            formula: Some("=A1+1".to_string()),
+            ..Default::default()
+        };
+        sheet.put_cell(1, 0, cell); // A2 formula, no recalc -> empty computed
+        sheet.set_cell(9, 0, "=SUM(A1:I9)");
+        assert_eq!(sheet.get_display_value(9, 0), "21"); // 10 + 11
+    }
+
+    // ── Number-format parse failures keep raw text ──────────────────────
+
+    /// Non-numeric cells formatted as Integer/Decimal/Percent keep their
+    /// raw text (the parse-failure branch of `write_display_value`).
+    #[test]
+    fn non_numeric_formatted_cells_keep_raw_text() {
+        let sheet = SpreadsheetData::default();
+        for fmt in [
+            NumberFormat::Integer,
+            NumberFormat::Decimal1,
+            NumberFormat::Decimal2,
+            NumberFormat::Percent,
+        ] {
+            assert_eq!(sheet.display_value_from_cell(&cell_with("n/a", fmt)), "n/a");
+        }
+    }
+
+    // ── Demo constructors ───────────────────────────────────────────────
+
+    #[test]
+    fn demo_q3_and_roi_construct_non_empty_sheets() {
+        let q3 = SpreadsheetData::demo_q3();
+        assert!(!q3.cells.is_empty());
+        let roi = SpreadsheetData::demo_roi();
+        assert!(!roi.cells.is_empty());
+    }
+
+    /// A formula referenced through another formula that fails to parse on
+    /// the recursive path propagates the parse error instead of panicking.
+    #[test]
+    fn parse_fallback_error_propagates() {
+        let mut sheet = SpreadsheetData::default();
+        let cell = CellData {
+            value: "=1+".to_string(),
+            formula: Some("=1+".to_string()),
+            ..Default::default()
+        };
+        sheet.put_cell(0, 1, cell); // B1 invalid formula, no cached AST
+        sheet.set_cell(0, 0, "=B1"); // A1 evaluates B1 through the parse fallback
+        assert!(sheet.get_display_value(0, 0).starts_with("#ERROR!"));
+    }
+
+    /// A formula cell inside a large range that carries a cached AST (but no
+    /// computed value yet) is evaluated via the cached-AST branch.
+    #[test]
+    fn large_range_cached_ast_branch() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "10");
+        let cell = CellData {
+            value: "=A1+1".to_string(),
+            formula: Some("=A1+1".to_string()),
+            cached_ast: formula2::parse("A1+1").ok(),
+            ..Default::default()
+        };
+        sheet.put_cell(0, 1, cell); // B1 formula w/ cached AST, empty computed
+        sheet.set_cell(9, 0, "=SUM(A1:I9)");
+        assert_eq!(sheet.get_display_value(9, 0), "21"); // 10 + 11
+    }
+
+    /// A formula cell inside a large range that fails to parse propagates the
+    /// error out of the range evaluation.
+    #[test]
+    fn large_range_parse_error_propagates() {
+        let mut sheet = SpreadsheetData::default();
+        sheet.set_cell(0, 0, "5");
+        let cell = CellData {
+            value: "=1+".to_string(),
+            formula: Some("=1+".to_string()),
+            ..Default::default()
+        };
+        sheet.put_cell(0, 1, cell); // B1 invalid formula inside the range
+        sheet.set_cell(9, 0, "=SUM(A1:I9)");
+        assert!(sheet.get_display_value(9, 0).starts_with("#ERROR!"));
+    }
+}