feat(client,host): PyroWave Apple Metal decoder + per-mode bitrate pin

- clients/apple: native Metal wavelet decoder + compute shaders (Phase 5),
  decoding PyroWave without embedding MoltenVK.
- pf-client-core: plumb user_flags/completeness through Decoder::decode_frame
  so the PyroWave backend parses chunk-aligned + partial AUs; gate the param's
  unused-warning to exactly the non-pyrowave builds (fixes -D warnings on the
  featureless Linux client build).
- punktfunk-host: on a mid-stream mode switch, re-resolve the "Automatic"
  PyroWave bitrate for the new mode's ~1.6 bpp operating point (explicit rates
  and H.26x ABR stay put); reject sub-128px PyroWave modes before the encoder
  rebuild instead of after the ack.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/encode/linux/pyrowave.rs

@@ -1172,7 +1172,7 @@ mod tests {
         buf.row_stride_in_bytes = [w as usize, (w / 2) as usize, (w / 2) as usize];
         buf.plane_size_in_bytes = [y.len(), cb.len(), cr.len()];
         assert_eq!(
-            pw::pyrowave_decoder_decode_cpu_buffer_synchronous(dec, &mut buf),
+            pw::pyrowave_decoder_decode_cpu_buffer_synchronous(dec, &buf),
             pw::pyrowave_result_PYROWAVE_SUCCESS
         );
         pw::pyrowave_decoder_destroy(dec);