docs: TESTING.md should mandate test updates alongside feature/fix PRs

[voidfreud]

Context

CONTRACT.md mandates "every PR that changes observable surface amends CONTRACT.md in the same commit." A similar discipline is needed for tests.

Proposed

Add to `docs/TESTING.md` §1:

Tests are part of every PR. Any PR that:

• adds a feature → adds a happy-path + at least one edge-case test
• fixes a bug → adds a regression test named `test_issue__` (per meta: regression-naming CI check (closed bugs must have test_issue_<n>) #240)
• changes observable behaviour → updates affected fast-lane tests
• changes integration behaviour → updates `tests/integration-baseline.json` in the same PR or a tight follow-up

A PR with new code but no test delta is rent-dodging. Reviewer must ask: why is there no test change?

Why TESTING.md and not CONTRACT.md

CONTRACT governs runtime; TESTING.md governs verification discipline. Mirroring CONTRACT's "amend in same commit" for tests makes the two docs interlocking.

Priority

Medium. Policy-level, cheap, prevents drift.

Related: #240 (regression-naming CI check — would enforce this automatically).

[voidfreud]

Fixed in PR #390 (docs/TESTING.md — rules). New §1.1 "Testing discipline" mandates test deltas alongside feature/fix PRs: feature→tests, bug→regression test per #240, behaviour change→test updates, integration change→baseline refresh.