fix: invalidate Cursor cache after missing-createdAt fix

[lincolnwan]

Summary

• Bump Cursor result cache version from 3 to 4 so stale v3 caches are invalidated.
• Respect CODEBURN_CACHE_DIR for Cursor result cache, matching docs and enabling isolated cache tests.
• Add a regression test for old v3 Cursor caches created before the missing-createdAt fix.

Why

This follows up on fe2e622 ("Skip Cursor bubble rows that lack a createdAt timestamp") from PR #321; it invalidates caches generated before that parser fix.

That parser fix made fresh parsing skip Cursor bubble rows without createdAt, but existing ~/.cache/codeburn/cursor-results.json files could still contain v3 cached calls timestamped with parse time.

If the Cursor DB mtime/size did not change, CodeBurn would continue trusting that stale cache and still report phantom current-day usage.

Testing

• npm test -- tests/cursor-cache.test.ts tests/providers/cursor.test.ts tests/providers/cursor-bubble-dedup.test.ts tests/providers/cursor-workspace-breakdown.test.ts
• npx tsc --noEmit

Notes

This does not change Cursor parsing logic. It only invalidates previously poisoned Cursor result caches and adds coverage for that behavior.

[ozymandiashh]

Rechecked #337 with a differential cache-invalidation repro, not just test/build output.

Behavior proof:

• Created an isolated temporary HOME, so no real local cache was touched.
• Created a dummy state.vscdb.
• Created a synthetic stale ~/.cache/codeburn/cursor-results.json with:
  • version: 3
  • matching dbMtimeMs and dbSizeBytes
  • one cached Cursor call timestamped 2099-01-01T00:00:00.000Z

Baseline on current pre-fix behavior:

{
  "cacheAccepted": true,
  "callCount": 1,
  "firstTimestamp": "2099-01-01T00:00:00.000Z",
  "firstDeduplicationKey": "cursor:stale-v3-proof"
}

Result on #337:

{
  "cacheAccepted": false,
  "callCount": 0,
  "firstTimestamp": null,
  "firstDeduplicationKey": null
}

Additional validation:

• npx vitest run tests/cursor-cache.test.ts tests/providers/cursor.test.ts tests/providers/cursor-bubble-dedup.test.ts tests/providers/cursor-workspace-breakdown.test.ts - 22/22 tests passed.
• npx tsc --noEmit --pretty false - passed.
• npm run build - passed.
• git diff --check origin/main...HEAD - passed.
• Claude Opus 4.7 effort max review - PASS.
• Gemini 3.1 Pro Preview review - PASS.

Review note:

• This PR correctly fixes the stale v3 cache problem after the missing-createdAt Cursor bubble-row parser fix.
• It is intentionally cache-only and does not change Cursor parsing logic.
• The separate Cursor agentKv / database-mtime timestamp attribution issue remains covered by Fix Cursor agentKv timestamp attribution #345.

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