docs(#3316): standing counter-test rule for error/fallback branches (#3317)

Strengthens TESTING-STANDARDS.md contract 6 (counter-tests for negative
space): a test on an error/fallback branch must assert the specific
degraded verdict the branch produces, not merely that the call did not
throw. Generalizes the liveness-vs-correctness finding from #3050 and
epic #3051 (closed, all 14 enumerated modules drained) into a standing
review expectation, now that the one-time enumeration is done.

Worked examples are the real pre-fix and post-fix shapes of
tests/worktree-safety.test.cjs's resolveWorktreeContext timeout
counter-test, verified directly against source.

Deliberately not lint-enforced: a pattern scan for fail-open shapes
scored 1 true positive against 3 false positives during #3051's own
measurement (source: epic #3051 body, Phase 3). Cross-linked from
CONTRIBUTING.md's QA Matrix Requirements so reviewers see it where
they already apply the matrix.

H4 of epic #3053.

Co-authored-by: sim <sim@local>

CONTRIBUTING.md

@@ -612,6 +612,8 @@ Happy-path tests are not enough for code that accepts user input, reads project
 
 See [`TEST-EXAMPLES.md`](TEST-EXAMPLES.md) for concrete demo tests that show these requirements in practice.
 
+**Standing rule for error/fallback branches:** feeding an adversarial input is not sufficient on its own — if the code degrades permissively instead of throwing, the test must assert the *specific* degraded verdict, not just that the call survived. See [`TESTING-STANDARDS.md` — "Standing rule: assert the degraded verdict"](TESTING-STANDARDS.md#standing-rule-assert-the-degraded-verdict-not-just-did-not-throw).
+
 Use this matrix when it applies to the changed surface:
 
 1. Happy path