Mutation-test a change

The traps behind each step are in docs/agents/tooling.md (Mutation testing with cargo mutants).

1. Mutate the diff, not whole files, into a private output directory:

   git diff "$(git merge-base origin/main HEAD)" > "$TMPDIR/d.patch"
   cargo mutants --in-diff "$TMPDIR/d.patch" --output "$TMPDIR/mutants-<topic>"

   Diff from the merge base, not from origin/main: once main moves past the branch point, a plain git diff origin/main carries main's new commits as deletions, and the run mutates lines the branch never touched.

   Do not pass --timeout, and keep fixtures small: one large frame turned an 8-minute run into 22.

2. Quote a run only after it prints N mutants tested in …; the outcomes file is rewritten while it runs.
3. Read survivors by grepping the function name in caught.txt and missed.txt. --re still queues field-deletion mutants, so its summary line overstates the misses.
4. A survivor may be a stale temp directory from an earlier run rather than a gap: a test writing under the temp dir clears its target first.
5. cargo-mutants never mutates a literal argument or deletes a statement. Hand-mutate the line a security fix rests on.
6. A surviving mutant in a guard nobody can reach is a design smell: remove the guard, or make it reachable and test it.