feat(pyrowave): Linux 4:4:4 encode — per-pixel CSC, full-res chroma, gate open

Phase 2 of design/pyrowave-444-hdr.md. can_encode_444(PyroWave) now returns
true on Linux, so a client that advertises VIDEO_CAP_444 (the 4:4:4 setting)
and prefers PyroWave negotiates a full-chroma wavelet session end to end
(the client decoder side landed in 5eb930e7).

- rgb2yuv444.comp: the 4:4:4 twin of rgb2yuv.comp — one invocation per pixel,
  full-res interleaved RG8 CbCr, no box filter/siting, byte-identical BT.709
  limited coefficients; compiled .spv committed (glslangValidator -V, matches
  the existing shader's toolchain).
- Encoder: chroma-conditional pyrowave create (open + reset), full-res chroma
  plane + views, per-pixel dispatch, 4:2:0-only even-dims check.
- Tests: decode oracle grows a 4:4:4 mode (YUV444P CPU readback);
  pyrowave_smoke_444 round-trips plane means AND drives the busy test card at
  the ~2.6 bpp operating point asserting in-budget + run-to-run deterministic
  AU sizes — the exact regime that silently corrupted before the vendored
  payload_data fix (patches/0001), so this doubles as its regression test.

Verified on .21 (RTX 5070 Ti): clippy -D warnings, host tests, and both GPU
smokes (pyrowave_smoke + pyrowave_smoke_444) green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-encode/src/lib.rs

@@ -871,10 +871,10 @@ pub fn can_encode_444(codec: Codec) -> bool {
     use std::sync::{Mutex, OnceLock};
     if codec == Codec::PyroWave {
         // PyroWave does its own RGB→YCbCr CSC (capture always hands it a full-chroma source),
-        // so 4:4:4 needs no GPU encode probe — only the full-res-chroma CSC variant, which
-        // hasn't landed yet (design/pyrowave-444-hdr.md: Phase 2 Linux, Phase 3 Windows).
-        // Flip per-OS when it does.
-        return false;
+        // so 4:4:4 needs no GPU encode probe — only the full-res-chroma CSC variant:
+        // `rgb2yuv444.comp` on Linux (landed, design/pyrowave-444-hdr.md Phase 2); the
+        // Windows `BgraToYuvPlanes` twin is Phase 3.
+        return cfg!(target_os = "linux");
     }
     if codec != Codec::H265 {
         return false;