perf(producer): cache transfer-converted hdr image buffers per render job - #384

Summary

Add HdrImageTransferCache — a per-render-job bounded LRU keyed by (imageId, targetTransfer) — so static HDR image layers whose source transfer differs from the render's effective transfer (PQ↔HLG) are converted once per job instead of once per composited frame.

Why

Chunk 8B of plans/hdr-followups.md. blitHdrImageLayer was running Buffer.from + convertTransfer on every composited frame, even though the converted buffer is identical for the entire job. For a multi-second comp at 30 fps this is hundreds of redundant transfer conversions on the hot path.

What changed

• New packages/producer/src/services/hdrImageTransferCache.ts — bounded LRU keyed by (imageId, targetTransfer) that owns the converted HDR rgb48 buffer for static HDR image layers:
  • Same-transfer requests return the source buffer untouched (zero copy).
  • Cross-transfer requests pay one Buffer.from + convertTransfer on first miss, reuse the cached copy on every subsequent frame.
• Wired into renderOrchestrator.ts via HdrCompositeContext.hdrImageTransferCache, instantiated once per render job, and consumed by blitHdrImageLayer on both the main composite path and the transition path.

Test plan

• packages/producer/src/services/hdrImageTransferCache.test.ts — 12 tests:
  • hit/miss semantics
  • distinct keys per image and per target transfer
  • LRU eviction + promotion
  • maxEntries=0 passthrough
  • source-buffer immutability for cached entries
  • invalid options
• Re-ran the Chunk 8A HDR benchmark — for the hdr-regression fixture (which has cross-transfer image layers) the cache hits 100% after the first frame; for HDR fixtures without cross-transfer images the same-transfer passthrough is a no-op.

Stack

Chunk 8B of plans/hdr-followups.md. Sits on top of Chunk 8C (logger gating) and Chunk 8A (benchmark harness) so the win is measurable.