Instruction file imported from Bellanco/myGameList (.github/instructions/tests.instructions.md). Copyright stays with the author.

Tests (tests/)

• Runner is Vitest (globals: true, jsdom environment). Files: tests/**/*.test.ts (and colocated src/**/*.test.ts).
• Layout: tests/unit/ (pure logic — CRDT merge, sanitize, sync), tests/integration/ (cross‑module), tests/e2e/ (smoke). npm run test runs unit + src; npm run test:all adds integration + e2e.
• Use the global describe/it/expect/vi (no imports needed thanks to globals: true).
• The merge/sync logic is the highest‑value thing to test — when you touch syncRepository.ts / syncMachineRepository.ts, add or extend tests there (tests/unit/syncRepository.test.ts, tests/unit/syncMachineRepository.test.ts).
• Prefer testing real exported functions over reimplementing logic. Mock network (Gist/Firestore) at the repository boundary; don't hit real APIs.
• Cover edge cases: empty/legacy data shapes, conflicting _ts, tombstones in deleted[], ETag 304 paths.
• Keep tests deterministic — no real timers/dates/network. Match existing test style.

Verify: npm run test (or npm run test:coverage).