OUT-2: Opus (CELT) encoder (effectcraft-opusenc)

New L0 crate effectcraft-opusenc: clean-room RFC 6716 CELT-only encoder
(fullband 48 kHz, 20 ms packets, mono/stereo as dual L/R stereo, constant
packet size from the bitrate) with the RFC 7845 OpusHead and pre-skip.
Forward low-overlap MDCT, inter/intra coarse energy, fine/final energy,
the decoder's exact allocation, recursive band splitting, PVQ search and
codeword indexing, silence frames.

crates/export depends on it; filmcraft-opus (dev) is the round-trip oracle
in tests/opus_roundtrip.rs, with ffmpeg reading an in-test Ogg Opus file
as a second oracle when installed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

xtask/src/main.rs

@@ -24,6 +24,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("geom", 0),
     ("color", 0),
     ("testkit", 0),
+    ("opusenc", 0),
     ("raster", 1),
     ("keyframe", 1),
     ("path", 1),