Mirror of the CAD engine harness for the doc crate, minus the shim
gymnastics (doc-engine depends only on serde/serde_json, so it
instruments directly): an isolated toolchain + cargo + target dir under
one mktemp directory, removed by a shell trap on every exit path;
nothing enters the host, the workspace target/, or $HOME. Runs the unit
tests plus tests/materialize.rs under -C instrument-coverage, enforces
a 96% total-lines floor against a 99.00% baseline plus per-file floors
(losing one module's tests must not hide in the total), and with
KEEP_COVERAGE=1 writes the uncovered-line listing that makes adding
branch tests directed rather than guesswork. COVERAGE.md records the
baseline, the exclusions, and the arms that are deliberately left
uncovered (defensive CRDT merge arms, one unreachable!, and the
Compensation::inverse arms unreachable through the public API).
