test(mutation): make anchors resilient and stale-anchor failures self-explaining

[willytop8]

Makes the mutation contract's anchors survive honest edits, and makes a detached anchor say so.

Why

mutation-contract.mjs locates each of its 67 mutants by exact string match. Any legitimate edit to
an anchored line silently detaches the mutant, and the contract then fails with:

expected exactly one mutation target, found 0

which reads like a safety regression rather than a stale anchor. This happened on #53: widening
REPLAY_SAFE_OPERATIONS to include the read-only children/status operations — a correct change —
broke the build, and the message gave no hint that the fix was a one-line anchor update.

Changes

• Anchors may be a RegExp, with to as a replacement string or a function of the match.
  Literal anchors still work and remain correct for pinned logic (a comparison, a call), where a
  change to the line genuinely should force someone to re-examine the mutant. Use a RegExp when the
  line carries a value that legitimately grows.
• REPLAY_SAFE_OPERATIONS is re-anchored on its declaration rather than its members, so the set
  can gain read-only operations without detaching the mutant. Verified against the current form, the
  pre-feat: add noInterruptOnUserMessage and noContinueWhileChildrenActive options #53 form that broke, a hypothetical future widening, and reformatted spacing — all resolve to
  exactly one match and still inject prompt.
• Stale-anchor failures explain themselves: they now name the file, state that this is a stale
  anchor rather than a failed property, say the guarded property is unverified until fixed, and point
  at the RegExp remedy. The >1 case gets its own distinct message.
• New guard: an anchor that matches but whose replacement leaves the source unchanged now fails
  loudly. Previously that would run the test against unmutated source and "pass" for the wrong
  reason — the mutant is supposed to make the test fail.

Checks

npm run test:mutation — 67/67 critical mutants killed.

The stale-anchor path was verified by deliberately detaching an anchor and reading the output, then
restoring and re-running to confirm 67/67.