feat(audio): end-to-end 5.1/7.1 surround across the native path + all clients

Adds negotiated 5.1/7.1 surround to the punktfunk/1 protocol and every client
(previously stereo-only):

- core: new shared `audio` layout table (LAYOUT_51/71 + identity multistream
  mapping, canonical wire order FL FR FC LFE RL RR SL SR); Hello/Welcome
  `audio_channels` negotiation via the trailing-byte back-compat pattern (old
  peers fall back to stereo); C-ABI `punktfunk_connect_ex6`,
  `punktfunk_connection_audio_channels`, and in-core multistream decode
  `punktfunk_connection_next_audio_pcm` for embedders without a multistream
  Opus decoder. Real-libopus channel-identity round-trip test.
- host: native audio thread captures + Opus-(multi)stream-encodes at the
  negotiated count (with a cross-session cached-capturer channel-mismatch fix);
  GameStream surround unified onto the safe `opus::MSEncoder`, dropping
  `audiopus_sys` (~4 unsafe blocks) and un-gating Windows GameStream surround;
  WASAPI loopback capture relaxed to 2/6/8 with the correct dwChannelMask.
- clients: Linux (PipeWire), Windows (WASAPI), Android (AAudio) decode via
  `opus::MSDecoder` + render multichannel; Apple decodes in-core to PCM →
  AVAudioEngine with an explicit wire-order channel layout; each gains a
  Stereo/5.1/7.1 setting. `punktfunk-probe --audio-channels N` is the headless
  validator.

Verified on Linux: core/host/linux/probe test suites + the Android Rust
(cargo-ndk) build, clippy -D warnings, and rustfmt all green. Windows/Apple
builds, all on-glass checks, and the live native loopback are pending (CI / a
free box).

Also lands the concurrent in-tree HEVC 4:4:4 host work (PUNKTFUNK_444): it
shares the same touched files (quic.rs, punktfunk1.rs, encode/*, ...) and so
cannot be committed separately from the surround changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/host.env.example

@@ -48,6 +48,12 @@ PUNKTFUNK_ZEROCOPY=1
 #PUNKTFUNK_INPUT_BACKEND=libei      # wlr | libei | gamescope | uinput
 #PUNKTFUNK_FEC_PCT=20               # video FEC overhead percent
 #PUNKTFUNK_PERF=1                   # per-stage timing logs
+# Full-chroma 4:4:4 (HEVC Range Extensions) — sharper text/desktop, no chroma loss. Honored only on
+# the punktfunk/1 native path when the client advertises 4:4:4 AND the GPU supports it (probed; else
+# the session stays 4:2:0). HEVC-only; independent of 10-bit. NVENC (NVIDIA) is the validated path;
+# VAAPI/AMF/QSV decline (4:2:0). GameStream/Moonlight always stays 4:2:0.
+#PUNKTFUNK_444=1
+#PUNKTFUNK_10BIT=1                  # HEVC Main10 / HDR (when the client advertises 10-bit)
 #RUST_LOG=info
 # Management API bearer token. The mgmt API is HTTPS + token-authenticated ALWAYS (even on
 # loopback); if unset it is auto-generated + persisted to ~/.config/punktfunk/mgmt-token (which the