test(hdr-regression): tighten Window C maxFrameFailures budget after Chunk 1 fix (#369)

## Summary

Tighten `hdr-regression` Window C `maxFrameFailures` from 30 → 5 now that Chunk 1 (opacity pipeline) has landed.

## Why

Window C (direct `<video>` opacity tween) was previously listed as a known failure with a `maxFrameFailures` budget of 30 to absorb expected drift until Chunk 1 landed. After the Chunk 1 fix, the regression test passes against the existing golden with **0 failed frames**. Tightening the budget catches any future drift in the opacity path immediately rather than letting up to 30 broken frames slip through.

## What changed

- `tests/hdr-regression/meta.json`: `maxFrameFailures` 30 → 5 (small budget remains for HEVC encoder noise).
- `tests/hdr-regression/README.md`: updated to mark Window C as fixed and note the tightened budget.

The HEVC encoder is byte-deterministic and the opacity fix doesn't perturb pixels at the PSNR ≥ 28 checkpoint threshold, so regenerating the golden produces byte-identical output. The golden is therefore unchanged. Window F (transform + border-radius) remains pending Chunk 4; its broken state is currently baked into the golden, so the suite is green and Chunk 4's regen will catch any drift.

## Test plan

- [x] `bun run test --filter hdr-regression` — passes with 0 failed frames at the new budget.

## Stack

Follow-up to Chunk 1 (opacity pipeline). Reviewable separately so the golden churn (none in this case) is decoupled from the code fix.

packages/producer/tests/hdr-regression/meta.json

@@ -3,7 +3,7 @@
   "description": "HDR10 (BT.2020 PQ) regression suite. Four windows (A–D) cover the highest-value HDR compositing shapes: direct <video> opacity tween (A), z-order DOM/HDR/DOM sandwich (B), transform + border-radius (C), and a shader transition between HDR video and HDR PQ image (D). 10s / 300 frames at 30fps.",
   "tags": ["regression", "hdr"],
   "minPsnr": 28,
-  "maxFrameFailures": 30,
+  "maxFrameFailures": 5,
   "minAudioCorrelation": 0,
   "maxAudioLagWindows": 1,
   "renderConfig": {