fix(core): let CODEC_PYROWAVE survive the Welcome decode whitelist

Found by the first live session on .21: the host negotiated PyroWave
and put codec=8 on the wire, but Welcome::decode's codec whitelist
(H264/AV1, else HEVC — the corrupt-byte guard) folded it to HEVC, so
the client opened an FFmpeg HEVC decoder against wavelet AUs. Roundtrip
test now pins the pyrowave byte (and that a genuinely unknown future
bit still folds to the HEVC default).

With the fix the Phase-2 exit session runs END TO END on .21
(host + session client on one box, host capturing the GNOME virtual
display, client presenting into a headless weston):
  negotiated codec=PyroWave (adv 0x0f) → PyroWave encoder (CPU-capture
  path — this box's Mutter+NVIDIA rejects the LINEAR-dmabuf offer) →
  wire → PyroWave decoder on the presenter's device → planar CSC.
  Static desktop: stable 60 fps, e2e 2.1-4.1 ms p50 (p95 <= 6 ms),
  decode 0.2-0.6 ms, vs HEVC/NVENC-direct baseline 2.1 ms — parity at
  idle. Full-window motion: 60 fps at ~80 Mb/s all-intra (HEVC ~7),
  decode still sub-ms, zero decode errors or keyframe-request chatter
  across every run. Deeper motion/loss characterization needs a
  dmabuf-accepting host box (this one is capped by the CPU capture
  path).

Also retires the stale "no shipping client decodes this" wording in
the host encoder/dispatch logs — the negotiation exists now.

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

crates/punktfunk-host/src/encode.rs

@@ -674,9 +674,10 @@ fn open_video_backend(
                 {
                     tracing::warn!(
                         ?codec,
-                        "PUNKTFUNK_ENCODER=pyrowave: EXPERIMENTAL all-intra wavelet stream — \
-                         clients without a PyroWave decoder (all of them until CODEC_PYROWAVE \
-                         lands) cannot display it"
+                        "PUNKTFUNK_ENCODER=pyrowave forces the all-intra wavelet stream \
+                         regardless of the negotiated codec — only a pyrowave-feature client \
+                         that ALSO preferred CODEC_PYROWAVE can display it (lab override; \
+                         normal sessions negotiate it instead)"
                     );
                     pyrowave::PyroWaveEncoder::open(width, height, fps, bitrate_bps)
                         .map(|e| (Box::new(e) as Box<dyn Encoder>, "pyrowave"))