Fix iOS audio: app-owned manual audio session for stable playout

[MaxHeimbrock]

This is just the result of an investigation

Problem

On iOS (Meet sample with PlatformAudio + a Unity AudioSource playing background music), the WebRTC ADM runs the shared AVAudioSession in automatic mode. As a result:

• Joining a call reroutes other app audio (ambient music) to the earpiece and attenuates it — WebRTC drops the DefaultToSpeaker option on reconfigure, and VoiceChat mode is receiver-biased.
• Hanging up deactivates the shared session (setActive:NO) out from under Unity/FMOD, so Unity audio dies and never recovers on rejoin.
• The session is left stuck in PlayAndRecord/VoIP forever — LiveKit_RestoreDefaultAudioSession existed but was never called.

Fix

Put WebRTC's RTCAudioSession into manual mode and have the app own the session. Entirely Unity-side (no Rust/FFI rebuild) — RTCAudioSession is reachable from the plugin (unprefixed class, already linked into liblivekit_ffi), and LiveKitAudioSession.mm is compiled by Xcode.

• Hold one permanent activation. RTCAudioSession ref-counts activation, so WebRTC's per-call setActive:NO on hang-up only decrements the count and never deactivates the hardware session → Unity audio survives.
• VideoChat mode routes to the loudspeaker by default (while honoring connected wired/Bluetooth headphones) → no more earpiece.
• Gate the VPIO unit via isAudioEnabled around connect/disconnect. Defaults to enabled so call audio works out of the box; disabled on hang-up stops the unit while the session stays alive.
• Restore the session on PlatformAudio.Dispose — balances the constructor so the session isn't left in PlayAndRecord.

Changes

• Runtime/Plugins/iOS/LiveKitAudioSession.mm — manual-mode RTCAudioSession (reached via NSClassFromString + a protocol-typed id, with an AVAudioSession fallback), VideoChat mode, held activation, new LiveKit_SetAudioEnabled, rewritten restore.
• Runtime/Scripts/Audio/PlatformAudio.cs — public SetSessionAudioEnabled(bool) + restore-on-Dispose.
• Samples~/Meet/Assets/Runtime/MeetManager.cs — enable on connect, disable on hang-up / disconnect.

Validation

• ✅ Meet sample compiles for iOS via IL2CPP (Unity 6000.3.10f1, Build SUCCEEDED, 0 error CS).
• ⚠️ Needs on-device validation — manual-mode integration touches libwebrtc internals. Verify routing stays on the loudspeaker across join / mute / unmute, and that Unity audio survives hang-up → rejoin. Watch the LiveKit: NSLogs; activationCount should never reach 0 during a connect → hang-up → reconnect cycle.

Known trade-off (unchanged, by design)

While a call is active, the VPIO voice-processing unit imposes iOS in-call volume (louder, not hardware-mutable to zero); it reverts to normal media volume after hang-up. That's inherent to keeping hardware echo cancellation — only disabling VPIO (a Rust rebuild that sacrifices AEC) removes it.

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