fix(ci): unbreak the Apple + Windows-client builds after the surround-audio merge

The 5.1/7.1 surround commit (75627c8) added in-core Opus, which broke two CI jobs
that the merge didn't touch:

  * Windows MSIX client: clients/windows/src/main.rs's headless `SessionParams`
    initializer was missing the new `audio_channels` field (the GUI path sets it
    from settings). Default the CLI/test path to stereo (2), matching trust.rs.
  * Apple xcframework (apple.yml + release.yml): in-core Opus decode pulls
    `audiopus_sys`, which builds a vendored *static* libopus via CMake when
    pkg-config finds no system Opus — keeping the xcframework self-contained (no
    runtime libopus.dylib on end-user Macs/devices). The self-hosted macOS runner
    lacked `cmake`; install it self-healing before every xcframework build.

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

clients/windows/src/main.rs

@@ -177,6 +177,8 @@ fn run_headless_cli(args: &[String], identity: (String, String)) {
         compositor: CompositorPref::Auto,
         gamepad: GamepadPref::Auto,
         bitrate_kbps,
+        // Headless CLI path (test/scripting) — stereo baseline; the GUI sources this from settings.
+        audio_channels: 2,
         mic_enabled: flag("--mic"),
         hdr_enabled: !flag("--no-hdr"),
         decoder,