feat(apple): PyroWave Phase 5 — native Metal decode on Mac / Apple TV / iPad (§4.7)

The Apple client now decodes PyroWave natively on the presenter's own MTLDevice —
no MoltenVK, no upstream C++ in the app. Completes and wires up the decoder whose
early working-tree snapshot rode along in 9127c346:

- MetalWaveletShaders.swift: wavelet_dequant + idwt hand-ported from the vendored
  GLSL (STORAGE_MODE 0 only; subgroup scans → 32-wide simdgroups; DCShift spec
  constant → function constant; precision-1 split: fp16 levels 0-1 / fp32 2-4).
- MetalWaveletDecoder.swift: Swift reimplementation of push_packet/decode_packet
  incl. the Phase-4 chunk-aligned window walk (FRAG chains, zeroed missing shards,
  the >half-blocks partial rule), init_block_meta's block-index space, and the
  42-dequant + 13-idwt dispatch structure with encoder-boundary barriers. SOF-dims
  changes rebuild the size-dependent resources, which is also the mid-stream
  resize path. Ring of 4 output plane sets on the presenter's queue.
- Presenter: pf_frag_planar (3xR8, the planar_csc.frag twin) + renderPlanar with
  a shared present tail; ReadyFrame carries an image enum (.video | .planar).
- Stage2Pipeline: a dedicated PyroWave pump — no VideoToolbox machinery, no
  keyframe/re-anchor recovery (all-intra; partials render as localized blur by
  design), newest-frame-index staleness guard for late partials.
- Opt-in: "PyroWave (wired LAN)" codec entry (probe-gated, ≈A13 floor via a real
  kernel-compile probe), selecting it advertises + prefers the codec and forces
  the session SDR (HDR/10-bit/4:4:4 caps dropped, plan contract).
- Core ABI: punktfunk_connection_shard_payload() — the Welcome's negotiated shard
  payload, needed by native decoders to walk chunk-aligned AUs.
- Validation: golden fixtures generated by the host encoder + upstream's own
  decoder (pyrowave_dump_golden, RTX 5070 Ti); the Metal decode PSNR-matches at
  77-88 dB across all planes for dense AND chunk-aligned AUs, and a hole-punched
  partial still decodes. Parser unit tests cover the window walk, FRAG chains,
  broken chains, the half-blocks gate, and the block-index layout.

Tests: apple 134 green (mac; iOS/tvOS build), host 312 w/ pyrowave on .21,
core 148 w/ quic; clippy/fmt clean.

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

include/punktfunk_core.h

@@ -1684,6 +1684,18 @@ PunktfunkStatus punktfunk_connection_codec(PunktfunkConnection *c,
                                            uint8_t *out);
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Read the session's negotiated wire shard payload (the `Welcome`'s value, bytes). This is the
+// parse-window size of a [`USER_FLAG_CHUNK_ALIGNED`] AU (PyroWave datagram-aligned mode,
+// design/pyrowave-codec-plan.md §4.4): every `shard_payload`-sized window of the frame buffer
+// starts a fresh self-delimiting chunk. Clients that decode PyroWave natively (the Apple Metal
+// port) need it to walk those AUs; other codecs never need this.
+//
+// # Safety
+// `c` is a valid connection handle; `out` is NULL or writable for one `u32`.
+PunktfunkStatus punktfunk_connection_shard_payload(PunktfunkConnection *c, uint32_t *out);
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Send one input event to the host as a QUIC datagram (non-blocking enqueue).
 //