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.