docs(testing): the fourth instrument rule — a mutation that reddens arms it cannot reach

[lilyshen0722]

Follow-up to #1947, cut from its merge commit 6f966eec. Vera ruled it should be its own docs PR rather than folded into TASK-167's pass, and #1947 was already in the merge queue when the bullet was ready (a force-push to a queued branch is refused, which is the protection working).

What it adds

A fourth instrument rule, beside the three #1947 landed:

A mutation that reddens arms it cannot reach means the instrument did not run.

The two rules beside it are visible in the instrument's own output — an unapplied replace leaves an empty diff; a fixture that is not the real shape answers the wrong question. This one is visible only in the shape of the red: a mutation to one module reddening a suite that never imports it is not a cause, and the run has already said it is lying. The audit trail is the restore helper open(dest, 'w').write(open(src).read()), which truncates the destination before reading the backup; a missing backup left the module under test at zero bytes, and the symptom was a controller mutation reddening all six arms of an unrelated script suite. The tell was never the count — the reds had no causal path to the mutation. Fix: read the source before opening the destination for write, and assert the backup is non-empty.

Vera named this as a distinct class and supplied the second instance in the same night: a ts-jest compile error that printed Tests: 20 passed with exit 1.

Two probe corrections folded into the array-wrapper bullet, both hers:

• !! in a probe coerces undefined to false — three distinguishable states reduced to two, with the printed result still looking like a measurement. Her first measurement of isMongooseDocumentArray was reported as false for exactly this reason.
• constructor.name is not a shape control: it reads 'Array' for both a patched array and a plain one, so it fails silently where isMongooseArray discriminates.

Verification

• __tests__/unit/scripts/docReferences.test.js — 6/6 (the witness docs(testing): three instrument rules the TASK-166 gate earned — fixture shape, a mutation that never applied, and load-red #1947 established for this file: it checks every path and file:line reference in the docs, so a docs PR that moves a reference reddens here).
• Docs-only, one file, no code path touched. Re-check is a diff from this head plus the witness above.

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Gate follow-up (head fee86ca9)

Vera's correction to her own contribution in the fourth bullet, applied. The bullet had said "a green summary from a run that never executed". The run did execute: 3 of 4 suites ran and accounted for all 20 passes; the fourth never compiled, so the summary was complete for the suites that ran and silent about the one that did not. The accurate form is now in the document, together with the operational half that follows from it: a suite that fails to run shows up as a missing contribution, not as a red test, so the check is the count against the baseline (91 → 20) rather than a scan for failures. That is a sharper trap than "never executed" — the number was internally consistent, which is exactly why it read as a normal green.

Re-check: this is one sentence in backend/TESTING.md, and the witness is docReferences 6/6.