T3: golden-image tests for the renderer + GPU parity; fix stale GPU uploads

crates/golden (test-only, L3): nine procedural 320x180 scenes (transform
and opacity, blend modes, Gaussian Blur, Lumetri basic, Crop, Cross
Dissolve / Dip to Black / Wipe mid-transition, timecode/clip-name text)
rendered on the CPU and compared with committed goldens (PSNR >= 45 dB,
max <= 12, p99 <= 2); FILMCRAFT_BLESS=1 regenerates them with
attribution sidecars. testkit::golden adds PNG I/O, diff metrics and
failure images. The same scenes are composited on the GPU and compared
with the CPU.

GPU parity exposed a real bug: GpuCompositor cached uploads by
pixel-buffer address without owning the buffer, so a new frame
allocated at a freed frame's address was drawn with a stale texture
(whole wrong frames, PSNR ~12 dB). The cache now keeps the buffer
alive; regression test upload_cache_keeps_buffers_alive.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

xtask/src/main.rs

@@ -46,6 +46,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("interchange", 2),
     ("render", 3),
     ("gpu", 3),
+    ("golden", 3),
     ("scopes", 3),
     ("playback", 3),
     ("export", 3),