New CI policy gate that detects when a PR changes source file Y but the test siblings of Y's consumers are not in the same diff. Prevents the 'forgotten sibling test' pattern (7 documented occurrences in PR #9529).
- Extracts shared import resolution library (resolveImport, sourceDepsOf) from build-test-impact-map.mjs
- Creates check-forgotten-sibling-tests.mjs gate with consumer-walk logic
- Adds forgotten-sibling-allowlist.json for false positive mitigation
- Wires into pr-test-policy CI job and adds npm script
- Documents in QUALITY_GATES.md
- 17 TDD unit tests for isSourceFile, resolveBase, testSiblingOf, isAllowlisted, findConsumers