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>

clients/linux/src/ui_settings.rs

@@ -140,6 +140,16 @@ pub fn show(
     input.add(&inhibit_row);
 
     let audio = adw::PreferencesGroup::builder().title("Audio").build();
+    let surround_row = adw::ComboRow::builder()
+        .title("Audio channels")
+        .subtitle("Request stereo or surround (the host downmixes if its output has fewer)")
+        .model(&gtk::StringList::new(&[
+            "Stereo",
+            "5.1 Surround",
+            "7.1 Surround",
+        ]))
+        .build();
+    audio.add(&surround_row);
     let mic_row = adw::SwitchRow::builder()
         .title("Stream microphone")
         .subtitle("Send the default input device to the host's virtual microphone")
@@ -170,6 +180,11 @@ pub fn show(
         compositor_row.set_selected(comp_i as u32);
         inhibit_row.set_active(s.inhibit_shortcuts);
         mic_row.set_active(s.mic_enabled);
+        surround_row.set_selected(match s.audio_channels {
+            6 => 1,
+            8 => 2,
+            _ => 0,
+        });
     }
 
     let dialog = adw::PreferencesDialog::new();
@@ -186,6 +201,11 @@ pub fn show(
             .to_string();
         s.inhibit_shortcuts = inhibit_row.is_active();
         s.mic_enabled = mic_row.is_active();
+        s.audio_channels = match surround_row.selected() {
+            1 => 6,
+            2 => 8,
+            _ => 2,
+        };
         s.save();
     });
     dialog.present(Some(parent));