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/pf-client-core/src/video.rs

@@ -628,6 +628,11 @@ impl Decoder {
     pub fn decode_frame(
         &mut self,
         au: &[u8],
+        // Only the PyroWave backend reads the flags; without that feature the param is unused.
+        #[cfg_attr(
+            not(all(target_os = "linux", feature = "pyrowave")),
+            allow(unused_variables)
+        )]
         user_flags: u32,
         complete: bool,
     ) -> Result<Option<DecodedImage>> {