fix(engine,shader): handle matrix3d transforms and hide non-first scenes (#374)

## Summary

Two correctness fixes in the HDR transform & clipping pipeline: `parseTransformMatrix` now handles `matrix3d(...)` (GSAP's default `force3D: true`), and shader-transitions sets every non-first scene to `opacity: 0` at `t=0` so the engine doesn't over-composite at the start.

## Why

`Chunk 4` of `plans/hdr-followups.md`. Transform extraction and border-radius computation existed but were dead — an HDR video with `rotation: 45` rendered un-rotated, and 3-scene compositions ghosted at `t=0` because every scene defaulted to CSS `opacity: 1` and contributed to the first frame.

## What changed

**Matrix3d support in `parseTransformMatrix`.** `DOMMatrix.toString()` emits `matrix3d` whenever any ancestor in the chain has used a 3D transform — most importantly GSAP's default `force3D: true`, which converts `translate(...)` into `translate3d(..., 0)`. Without this, every GSAP-driven transform was silently dropped during HDR compositing because `videoFrameInjector.getViewportMatrix()` would return `matrix3d(...)` and the blit path would parse it as `null` and fall back to identity. The 16-value column-major form is converted to its 2D affine projection (indices 0, 1, 4, 5, 12, 13 → m11, m12, m21, m22, m41, m42); Z, perspective, and out-of-plane rotation components are dropped.

**Initial-state opacity in `initEngineMode`.** The browser preview branch uses a GL canvas overlay during transitions, so scene opacity at `t=0` doesn't matter visually. The engine branch reads scene opacity directly via `queryElementStacking()` to decide which layers to composite. Without an explicit initial-state tween, every scene defaulted to CSS `opacity: 1` and contributed to the very first frame, causing ghosting/overlap until the first transition fired. `tl.set()` at position 0 anchors the initial state in the timeline graph so reverse seeks from inside a later transition restore it correctly.

These two fixes together make `el.transform` and `el.borderRadius` (already wired in Chunk 7A's `compositeHdrFrame`) actually flow through the GSAP-animated case, and keep the engine's per-frame compositing aligned with what the user sees in browser preview.

## Test plan

- [x] 6 new `alphaBlit.test.ts` cases (identity matrix3d, translate3d, scale + translate3d, rotateZ, malformed arg count, non-finite values).
- [x] Existing `hdr-regression` Window H already CSS-sets `#scene-b { opacity: 0 }` as a fallback; the new `tl.set` is redundant for that case but harmless and removes the need for compositions to remember the CSS workaround.
- [x] Manual: rotated HDR video (`rotation: 45`) appears rotated; `border-radius: 50%` clips to circle; 3-scene composition has no overlap at `t=0`.

## Stack

Chunk 4 of `plans/hdr-followups.md`. Window F of the regression suite documents the bug; the next PR in the stack tightens the `maxFrameFailures` budget to 0.

packages/producer/tests/hdr-regression/src/index.html

@@ -130,7 +130,7 @@
       data-width="1920"
       data-height="1080"
     >
-      <!-- Window A · Baseline HDR + direct opacity tween · 0.0–2.5s -->
+      <!-- Window A · Baseline HDR + direct opacity tween · 0.0–1.5s -->
       <video
         id="wa-video"
         class="clip hdr-video"
@@ -241,4 +241,4 @@
       window.__timelines["hdr-regression"] = tl;
     </script>
   </body>
-</html>
+</html>