perf(pyrowave): pool encoder scratch buffers + fix client parser O(n²) — lift the 2.5 Gbps wall

Real-world PyroWave streaming maxed ~2.5 Gbps with sagging fps while raw
transport does 4.8. Root-caused to serial per-frame paths at BOTH ends
(the transport was never the limit); this fixes the two dominant ones.

Host (vendored shim, patch 0004): pyrowave_encoder_encode_gpu_synchronous
allocated four Vulkan buffers (meta + bitstream, Device + CachedHost) on
EVERY frame. At 240 fps with MB-scale bitstreams that per-frame allocator
churn stalled the encode itself. Pool them on the encoder and reuse across
frames (recreate only on a size grow); the sizes are session-fixed, so it
is pure reuse after frame 1. On an RTX 4090 the 5120x1440 submit+fence-wait
drops ~15 ms -> ~1 ms, i.e. the host serial ceiling goes 64 -> 1025 fps
(444+HDR 44 -> 614). Safe under the synchronous encode model; re-validated
by pyrowave_win_smoke (Windows) and pyrowave_smoke/_444 (Linux). Applies to
both host encoder paths (they share the shim).

Client (Apple Metal decoder): WaveletBitstream.parse reserved the payload
buffer per packet (reserveCapacity(count + words), an exact realloc each of
~3000 packets/frame => O(n²)) and copied word-by-word. Reserve once up
front and memcpy each packet's coefficients in one shot (all Apple
platforms are little-endian, so the wire's LE u32s land verbatim; memcpy is
alignment-free). 5.44 ms -> 0.055 ms per 1.44 MB frame (25x); byte-identical
(parser unit tests + golden-frame PSNR unchanged).

Also:
- native.rs: PUNKTFUNK_PYROWAVE_MAX_MBPS caps PyroWave's open-loop Automatic
  bitrate pin for hosts on a constrained link (unset => no cap; an explicit
  client rate bypasses it). The pin is all-intra + ABR-off, so at a high
  pixel rate it can outrun the fabric (4:4:4+HDR 5120x1440@240 pins ~5.3
  Gbps, over a 5 GbE link) and the overshoot just becomes loss.
- pf-encode caps(): report the real opened chroma instead of a hardcoded
  4:2:0 default, so a genuine 4:4:4 session no longer trips the spurious
  "encoder chroma disagrees with the negotiated Welcome" warn. Also fix a
  latent Windows reset() that rebuilt at 4:2:0 for a 4:4:4 session.

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

crates/pyrowave-sys/vendor/pyrowave/PUNKTFUNK-VENDOR.txt

@@ -15,3 +15,13 @@ Local patches (crates/pyrowave-sys/patches/, re-applied on re-vendor):
     on the GPU → nondeterministic corrupt bitstreams/crashes at any bitrate.
     Found + validated 2026-07-18 (RTX 5070 Ti, 1080p/4K, 8/16-bit); to be
     reported upstream.
+
+  (0002-0003 are reserved by concurrent in-flight work and land separately.)
+
+  0004-encoder-buffer-pool.patch — pyrowave_c.cpp allocated four Vulkan buffers
+    (meta + bitstream, Device + CachedHost) on EVERY encode. At 240 fps with
+    MB-scale bitstreams that per-frame churn stalled the encode itself: on an
+    RTX 4090 the 5120x1440 submit+fence-wait was ~15 ms (~64 fps ceiling) and
+    dropped to ~1 ms (~1025 fps) once the buffers are pooled on the encoder and
+    reused. Safe under the synchronous encode model; re-validated by
+    pyrowave_win_smoke. Perf fix, not a correctness fix.