test(producer): add hdr-regression and hdr-hlg-regression test suites (#365)

## Summary

Replace the trivial `hdr-pq` and `hdr-image-only` tests with two consolidated, time-windowed regression suites that exercise the full HDR pipeline. These goldens are the safety net for every other PR in this stack.

## Why

The pre-existing HDR tests covered only a single full-bleed video or image with a static text label — none of the features that the HDR pipeline has to handle differently from SDR (opacity animation, z-ordered multi-layer compositing, transforms, border-radius clipping, shader transitions, multiple HDR sources, object-fit modes, mixed HDR+SDR layering, HLG transfer). This PR builds the missing safety net first so every subsequent fix can be proven correct.

## What changed

- New `packages/producer/tests/hdr-regression/` (PQ, BT.2020, ~20 s, 1080p, 8 windows A–H):
  - A: static baseline (HDR video + DOM overlay)
  - B: wrapper-opacity fade
  - C: direct-on-`<video>` opacity tween (documents the Chunk 1 bug)
  - D: z-order sandwich (DOM → HDR → DOM)
  - E: two HDR videos side-by-side (pins PR #289)
  - F: rotation + scale + border-radius (documents the Chunk 4 bug)
  - G: `object-fit: contain`
  - H: shader crossfade between HDR video and HDR image
- New `packages/producer/tests/hdr-hlg-regression/` (HLG, ARIB STD-B67, ~5 s, 2 windows A–B) — exercises the separate HLG LUT/OETF code path that previously had **zero** coverage.
- New `scripts/generate-hdr-photo-pq.py` synthesizes `hdr-photo-pq.png` with a cICP chunk for BT.2020/PQ/full.
- Removed `tests/hdr-pq/` and `tests/hdr-image-only/`.
- Updated `.github/workflows/regression.yml` HDR shard to run the new pair sequentially.
- All compositions follow the documented timed-element pattern (`data-start`, `data-duration`, `class="clip"` directly on each timed leaf — no wrapper inheritance).

## Test plan

- [x] Goldens generated with `bun run test:update --sequential`.
- [x] `ffprobe` confirms HEVC/yuv420p10le/bt2020nc/smpte2084 (PQ) and arib-std-b67 (HLG).
- [x] Suite green with `maxFrameFailures` budgets that absorb the documented Chunk 1 / Chunk 4 known-fails — tightened in follow-up PRs in this stack.

## Stack

Foundational PR for the HDR follow-ups stack (Chunk 0 of `plans/hdr-followups.md`). Every subsequent PR builds on this safety net.

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

@@ -0,0 +1,14 @@
+{
+  "name": "hdr-regression",
+  "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,
+  "minAudioCorrelation": 0,
+  "maxAudioLagWindows": 1,
+  "renderConfig": {
+    "fps": 30,
+    "workers": 1,
+    "hdr": true
+  }
+}