OUT-2: WebM export (VP9 intra encoder + alpha, Opus), WAV/AIFF audio-only outputs

- effectcraft-vp9enc (L0): VP9 profile 0 key-frame encoder from the VP9 bitstream spec
  (8x8 intra blocks, DC/V/H/TM, 4x4 DCT/ADST, lossless WHT at quality 100, tile columns);
  round-trips exactly through FilmCraft's VP9 decoder and ffmpeg.
- export: WebM muxer (EBML/Matroska spec) with VP9, VP9 alpha in BlockAdditional
  (AlphaMode 1), Opus (CodecDelay = pre-skip); WAV and AIFF outputs.
- OutputFormat WebM/Wav/Aiff in the Render Queue, renderQueue.add formats, CLI --format.
- Tests: FilmCraft decode-back (frames, size, pixels, audio RMS), ffprobe/ffmpeg oracles.

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

xtask/src/main.rs

@@ -23,6 +23,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("time", 0),
     ("geom", 0),
     ("color", 0),
+    ("vp9enc", 0),
     ("testkit", 0),
     ("opusenc", 0),
     ("raster", 1),