test(producer): extract frameDirMaxIndexCache to its own module and pin cross-job isolation

[vanceingalls]

Summary

Extract the frameDirMaxIndexCache from a private module-scoped Map inside renderOrchestrator.ts into its own frameDirCache.ts module, then add a 11-test bun:test suite that pins the cross-job isolation contract added in Chunk 5B.

Why

Chunk 9E of plans/hdr-followups.md. The cache lived as a private Map inside renderOrchestrator.ts, which made the cross-job isolation contract from Chunk 5B impossible to unit-test directly. Extracting it both makes the contract testable and reduces orchestrator complexity slightly.

What changed

• New packages/producer/src/services/frameDirCache.ts exposes getMaxFrameIndex / clearMaxFrameIndex / getMaxFrameIndexCacheSize (plus a test-only __resetMaxFrameIndexCacheForTests helper). Behavior is unchanged: callers still get the same module-scoped sharing inside a job, and renderOrchestrator's outer finally still clears every entry it registered so the cache cannot grow monotonically across renders.
• renderOrchestrator.ts: imports the new helpers, drops the unused readdirSync import, updates inline comments, and replaces two frameDirMaxIndexCache.delete sites with clearMaxFrameIndex.
• New frameDirCache.test.ts (bun:test, 11 tests) covering:
  • Reading the max index from a populated directory.
  • Ignoring filenames that don't match frame_NNNN.png (wrong ext, wrong prefix, wrong case, double extension, empty index group, same-named subdirectory).
  • Empty- and missing-directory paths returning 0 and being cached.
  • Intra-job invariant: subsequent readdir mutations not observed once cached.
  • clearMaxFrameIndex forcing a re-read; returns false for paths that were never cached.
  • Per-directory isolation when multiple directories are registered.
  • The cross-job contract from Chunk 5B: cache empty between well-behaved jobs, doesn't grow monotonically across 20 simulated renders with 3 HDR videos each (steady-state cache size stays at 3), and a buggy job that forgets to clear leaks exactly its own entries rather than affecting unrelated jobs.

Test plan

• frameDirCache.test.ts 11/11 pass.
• Existing producer tests unchanged.
• Behavior preserved: same module-scoped sharing inside a job, same outer-finally eviction.

Stack

Chunk 9E of plans/hdr-followups.md. Test-driven extraction; complements Chunk 5B.

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• docs(engine): document __name polyfill and add regression test #385
• perf(producer): cache transfer-converted hdr image buffers per render job #384
• perf(producer): gate per-frame debug meta via optional isLevelEnabled #383
• perf(producer): hdr benchmark harness — --tags filter, peak heap/RSS tracking, bench:hdr script #382
• test(producer): extract frameDirMaxIndexCache to its own module and pin cross-job isolation #381 👈 (View in Graphite)
• test(engine): cover spawnStreamingEncoder lifecycle and cleanup paths #380
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• test(hdr-regression): tighten Window C maxFrameFailures budget after Chunk 1 fix #369
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• refactor(shader-transitions): extract DEFAULT_DURATION and DEFAULT_EASE constants #367
• refactor(types): tighten type safety, dedupe HfTransitionMeta, prune dead LUT export #366
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• main

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