# ADR 002: Evaluator scope is finish-circle plus collision/proximity

Status: accepted (Phase 2 honesty pass).

## Context

The evaluator docs claimed per-variant simulation (roundabout exit arms,
sign stop-lines, priority yield). The code checked the finish radius plus
collision/proximity for five of nine variants; the payload fields
(`enter`, `exit_at`, `give_way`, `signs`, `yield_`, `stop`) were unread.

## Decision

Document exactly what each variant checks (`objective.rs` module docs)
and pin it with behavior-locking tests. The payload fields stay as
reserved data. A variant earns a real check only with geometry/query
plumbing plus tests in the same change — never docs first.

## Consequences

- Roundabout/Signs/Priority/Hazards/FreeDrive are finish-circle by
  design, not by neglect. Say so out loud (the module doc does).
- Reopen per variant when its check is implemented; each reopening must
  update the doc AND the locking test together.

## Amendment 2026-09-25 (practice-preview honesty, no new assessment)

- U-turn reversal is now course-RELATIVE heading (|relative| > 90°), not
  world forward-x (TRAFFIC-P1-02 partial; `uturn_uses_course_relative...`
  test). Turn zones/sequences/kerb remain reserved.
- Proximity is never labelled contact: priority/hazard fail text says
  "too close", never "collision" (invariant 7; adversarial tests pin it).
- `haz_child_run`, `haz_branch`, `haz_rain` now spawn their actor
  (TRAFFIC-P0-07 fixed; spawn-count test pins 1 each).
- Completion text for every variant says PRACTICE and never confirms an
  unobserved behaviour (`rule_adversarial` pins the banned strings).
- No variant is reopened as assessment: status stays PracticePreview for
  the whole catalog until TRAFFIC-08 curriculum sign-off lands.

## Amendment 2026-09-25b (stateful production engine, still practice)

- Production ticks go through `RuleEngine::step` (one-tick `evaluate`
  stays as the stateless, tested truth table sharing predicates and
  message constants with the engine):
  - speed needs 30 sustained over-limit ticks (grace) and 30 calm ticks
    to re-arm — one sampling artifact can no longer decide assessment;
  - lane/parking mistakes are edge-triggered per transition (no
    repeated-frame penalties); parking needs a 60-tick stopped dwell;
  - every transition lands in a bounded (4,096 + drop count) evidence
    trace explaining the result;
  - legal U-turns need actual reversal even at the finish marker.
- Engine, trace bounds, determinism, and dwell/grace boundaries are
  pinned by 7 unit tests. Variants without observability (roundabout
  exits, signs, gaps) remain finish-marker practice by design.