[ci] Test files in TS-vacuum — fixture drift not caught by gate (TD-foll-110)

[ignromanov]

Problem

__tests__/** files are currently in a TypeScript-checking vacuum:

Check	Sees test files?	Why
pnpm type-check:build (tsc --build)	❌ No	Each layer's tsconfig.json (e.g. src/entities/tsconfig.json:14) explicitly excludes __tests__/** for build perf
vitest	❌ Type-wise	Transpiles via esbuild, doesn't enforce strict types at runtime
IDE LSP (root tsconfig.json)	✅ Yes	But IDE ≠ CI

Result: test fixtures can drift from canonical types and CI stays green. Discovered concretely during PR #195 (GH#110) — InvoiceTemplate fixture used id/data/updatedAt instead of templateId/invoiceData/(no field). All gates green; only IDE flagged it. Fixed in commit 5bbc8b6.

Why It's Dangerous

• "tests pass = types correct" assumption is false. Fixture-shape bugs survive merge.
• New developers running pnpm test get false confidence.
• When fixture is later read by reachable code (refactor, new test), runtime undefined access.
• Erodes the architecture-as-contract guarantee — the canonical type registry is no longer enforced everywhere.

Proposed Fix

Add pnpm type-check:tests as a separate gate:

1. Create tsconfig.test.json at repo root, extending tsconfig.json, without the __tests__/** exclude. May need composite: false and noEmit: true.
2. Add npm script: "type-check:tests": "tsc --noEmit -p tsconfig.test.json".
3. Wire into CI alongside type-check:build and lint (parallelize: pnpm type-check:build & pnpm type-check:tests & pnpm lint & wait).
4. Document in CLAUDE.md and rules/testing.md.

Acceptance

• pnpm type-check:tests runs in CI on every PR
• Fails on type errors in __tests__/** (verify with a deliberate broken fixture in a draft PR)
• Documented in testing rules

Effort

S, ~1h. Low risk — purely additive gate.

References

• Discovered in PR refactor(creator-store): extract CREATOR_INITIAL_STATE as single source of truth (TD-11) #195 (GH#110)
• Fix commit: 5bbc8b6 test(creator-store): align InvoiceTemplate fixture with canonical type
• Test patterns: .ai/.claude/rules/testing.md