test: keep the mutation harness — scripts/mutation_check.py, with its own controls

Promotes the session-scratchpad harness that proved U7's twelve falsifiers into
a repo tool, on the operator's call. No version bump: test tooling and docs, no
production-code change, per the SemVer SKIP list.

A green test is not evidence. A test that has never seen its own defeating
change may pass under it too, forbidding nothing while reading as though it
forbids something. This repo shipped that three times — twice in one session,
and once an hour after writing the persistent-memory entry about it. Prose in a
memory file is not an instrument.

Tables live in tests/mutations/*.toml, one per unit, committed so a unit's
proofs are an artifact rather than terminal scrollback. Adding a unit means
adding a file, never editing the script. u7_navigation.toml was generated from
the harness that proved those twelve, not retyped, and every anchor was verified
against the source before it landed.

THE TOOL GETS ITS OWN POSITIVE AND NEGATIVE CONTROLS, which is the point. It
shipped two defects in one session, each of which made it report a falsifier
PROVED WITHOUT RUNNING IT, and both were found by accident rather than by
anything checking:

  no green baseline — a test that is ALREADY red reports red for every mutation
  thrown at it, so a broken assertion reads as a certified falsifier

  the bytecode cache — `< 2` -> `< 1` is byte-identical in size, and CPython
  validates a .pyc against the source's (mtime, size) at one-second granularity,
  so a mutation landing in the same second as the revert before it runs against
  cached bytecode; the tell was a verdict flipping between consecutive identical
  runs

tests/test_mutation_check.py now carries a control for each, plus the one
usually skipped: a KNOWN-VACUOUS falsifier the tool must catch. An instrument
that only ever sees unknowns cannot tell "nothing wrong here" from "I am blind",
and twelve `proved` lines from a blind instrument are worth nothing.

Also hardens the tool against itself: it writes to tracked source files, so the
restore is verified rather than assumed, and a .mutation-inflight marker makes a
run killed mid-mutation refuse the next start instead of silently measuring a
mutated tree.

648 tests green; 12/12 U7 falsifiers still proved.

.gitignore

@@ -6,3 +6,6 @@ __pycache__/
 booth-data/
 uv.lock
 graphify-out/
+
+# scripts/mutation_check.py crash marker — never committed
+.mutation-inflight