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/README.md

@@ -0,0 +1,37 @@
+# HDR Regression Suite
+
+HDR10 (BT.2020 PQ) regression suite with four back-to-back windows (A–D)
+covering the highest-value HDR compositing shapes. 10s / 300 frames at 30fps.
+
+## Windows
+
+| # | Window                              | Pipeline aspect under test                                                                                |
+| - | ----------------------------------- | --------------------------------------------------------------------------------------------------------- |
+| A | Baseline HDR + direct opacity       | HDR pass-through with a GSAP opacity tween directly on the `<video>` element.                             |
+| B | Z-order sandwich (DOM → HDR → DOM)  | Orange background, HDR video in the middle, blue overlay on top. Tests z-ordered layer compositing.       |
+| C | Transform + border-radius           | HDR `<video>` with `transform: rotate() scale()` + `border-radius` clipping. Tests affine blit pipeline. |
+| D | Shader transition (HDR → HDR image) | Shader transition between an HDR video and an HDR PQ image. Tests HDR image transfer cache + shader path. |
+
+## Fixtures
+
+- `src/hdr-clip.mp4` — short HEVC Main10 / BT.2020 PQ clip with a moving
+  bright gradient (see `NOTICE.md` for attribution).
+- `src/hdr-photo-pq.png` — 256x144 16-bit RGB PNG with a hand-injected `cICP`
+  chunk (primaries=BT.2020, transfer=SMPTE ST 2084, matrix=GBR, range=full).
+
+To regenerate the PNG fixture:
+
+```bash
+python3 packages/producer/tests/hdr-regression/scripts/generate-hdr-photo-pq.py
+```
+
+## Running
+
+```bash
+cd packages/producer
+bun run test hdr-regression
+bun run test:update hdr-regression
+```
+
+In CI it runs in the `hdr` shard alongside `hdr-hlg-regression`
+(see `.github/workflows/regression.yml`).