Add mutation testing for the state machine and engine

[bagowix]

Motivation

Coverage is enforced at 100% branch coverage (fail_under = 100), which proves
every line and branch executes — not that any assertion would notice if the
behaviour changed. For a state machine whose whole value is threshold arithmetic
and transition ordering, that gap matters: flipping >= to >, swapping a
threshold comparison, or dropping a state reset can easily leave a
100%-covered suite green.

Mutation testing closes that gap and produces a number worth publishing next to
the coverage badge.

Proposal

Add mutmut (3.x) as a dev-only tool, scoped to the modules where a surviving
mutant is a real bug:

• interlock/_state_machine.py (257 lines) — transitions, thresholds, probe
  admission;
• interlock/_engine.py (562 lines) — lock scope, dispatch, recording order.

Run it out of band, not on every PR: mutation runs are minutes-to-tens-of-
minutes, and the signal is a slowly-moving score, not a per-commit gate.

Acceptance criteria

• mutmut configured in pyproject.toml with the two modules above as the
  target and the fast subset of tests as the runner (exclude the
  wall-clock tests/test_examples.py and the extras integration tests).
• A documented local command in CONTRIBUTING.md.
• A scheduled (weekly) or manually dispatched workflow that runs mutation
  testing and surfaces the score; not a required PR check.
• Baseline score recorded in the issue or the repository so later runs have a
  comparison point.
• Every surviving mutant is either killed by a new test or documented as
  equivalent/intentional.
• The core stays dependency-free.

Non-goals

• Extending mutation testing to the integrations (interlock/integrations/) or
  the pipeline in the first pass.
• A hard mutation-score threshold as a merge gate.
• Replacing the coverage gate — the two are complementary.