test(producer): extract frameDirMaxIndexCache to its own module and pin cross-job isolation (#381)

## 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

- [x] `frameDirCache.test.ts` 11/11 pass.
- [x] Existing producer tests unchanged.
- [x] 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.

packages/engine/src/services/videoFrameExtractor.test.ts

@@ -134,8 +134,7 @@ describe.skipIf(!HAS_FFMPEG)("extractAllVideoFrames on a VFR source", () => {
       "-i",
       "testsrc2=s=320x180:d=10:rate=60",
       "-vf",
-      "drawtext=text='n=%{n}':fontsize=24:fontcolor=white:x=10:y=10:box=1:boxcolor=black@0.6," +
-        "select='not(between(n,30,89))*not(between(n,180,239))*not(between(n,330,389))*not(between(n,480,539))'",
+      "select='not(between(n\\,30\\,89))*not(between(n\\,180\\,239))*not(between(n\\,330\\,389))*not(between(n\\,480\\,539))'",
       "-vsync",
       "vfr",
       "-c:v",