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>

crates/punktfunk-host/src/config.rs

@@ -7,7 +7,7 @@
 //! **Goal-1 stages 1–2** (`design/windows-host-rewrite.md` §2.2): stage 1 stood this up; stage 2 migrated the
 //! genuinely-constant operator/dispatch knobs onto it (the dispatch-disagreement bug class: `idd_push`,
 //! `capture_backend`, `encoder_pref`, `render_adapter`, `no_wgc`, the vdisplay backend select — plus the
-//! plan-named `secure_dda`/`idd_depth`/`zerocopy`/`ten_bit` and the multi-site `perf`/`compositor`/
+//! plan-named `secure_dda`/`idd_depth`/`zerocopy`/`ten_bit`/`four_four_four` and the multi-site `perf`/`compositor`/
 //! `video_source`/`gamepad`). `SessionPlan` (stage 3) consumes it as the single owner of the
 //! capture/topology/encoder decision.
 //!
@@ -63,6 +63,10 @@ pub struct HostConfig {
     pub zerocopy: bool,
     /// `PUNKTFUNK_10BIT` — host policy gate for HEVC Main10 (only honored when the client also advertised 10-bit).
     pub ten_bit: bool,
+    /// `PUNKTFUNK_444` — host policy gate for full-chroma HEVC 4:4:4 (Range Extensions). Honored only
+    /// when the client also advertised 4:4:4, the codec is HEVC, and the GPU/driver supports a 4:4:4
+    /// encode (probed) — otherwise the session stays 4:2:0. Independent of `ten_bit` (chroma vs depth).
+    pub four_four_four: bool,
     /// `PUNKTFUNK_PERF` — per-stage timing instrumentation.
     pub perf: bool,
     /// `PUNKTFUNK_VIDEO_SOURCE` — GameStream video source select (`virtual` / `portal` / unset → synthetic).
@@ -112,6 +116,7 @@ impl HostConfig {
                 .unwrap_or(2),
             zerocopy: flag("PUNKTFUNK_ZEROCOPY"),
             ten_bit: flag("PUNKTFUNK_10BIT"),
+            four_four_four: flag("PUNKTFUNK_444"),
             perf: flag("PUNKTFUNK_PERF"),
             video_source: val("PUNKTFUNK_VIDEO_SOURCE"),
             compositor: val("PUNKTFUNK_COMPOSITOR"),