This PR significantly improves audio and video stream stability, particularly for iOS devices, by implementing proper pre-buffering, background/foreground handling, resource lifecycle management, and memory safety improvements.

🎵 Audio Stream Improvements (AudioStream.cs)

Pre-buffering Strategy

• Unified 200ms ring buffer for all platforms (removed platform-specific sizing)
• Smart 30ms pre-buffering: Wait for 30ms of data before starting playback to prevent underruns
• Automatic underrun detection and recovery: When buffer runs low (< 50% of requested samples), immediately output silence and wait for 30ms refill before resuming
• Result: Eliminates choppy audio during network jitter while maintaining low latency (~30-50ms in practice)

Background/Foreground Handling

• Clear ring buffer and reset priming state when app returns from background
• Prevents playing stale audio data after resume
• Ensures fresh 30ms buffer before resuming playback
• Fixes: Audio glitches and delays when resuming iOS apps from background

Memory Safety

• Fixed memory leak: Added using statements for AudioFrame and resampled frames
• Proper disposal ordering: Remove event delegates before disposing resources
• Added null checks in audio read callback to prevent crashes after disposal

🎤 Microphone Source Improvements (MicrophoneSource.cs)

iOS Audio Session Handling

• Proper pause handling: Stop microphone when app goes to background to release AVAudioSession
• Smart restart logic: Poll for microphone device availability (up to 2s timeout) instead of arbitrary delays
• Fixes: FigCaptureSourceRemote errors (-17281) and ArgumentNullException during background/foreground transitions

Component Lifecycle Management

• Use DestroyImmediate() to remove existing AudioSource/AudioProbe before adding new ones
• Fixes: Race conditions during quick stop/start cycles where Unity's deferred Destroy() caused component duplication
• Added null safety checks for GameObject validity before accessing components

Error Handling

• Graceful error handling with try-catch around Microphone.Start()
• Timeout-based polling (2s) for microphone readiness instead of indefinite WaitUntil()
• Re-check microphone authorization after background (permissions can change)
• Clear error logging for all failure cases

🎥 Video Stream Improvements (VideoStream.cs)

Frame Management

• Latest-frame-wins coalescing: Keep only the newest frame when Rust delivers bursts
• Separate pending/active frame buffers: Prevents race conditions between frame production (FFI) and consumption (Unity update)
• Lock-protected frame swapping: Thread-safe handoff between main thread and rendering

Resource Lifecycle

• Proper disposal of pending frames when stopping stream
• Immediate frame disposal when stream is not playing (prevents native memory growth)
• Fixed disposal ordering to prevent late callbacks from accessing disposed state
• Only allocate VideoFrame objects when FrameReceived event has listeners

🔧 Supporting Infrastructure

RingBuffer (RingBuffer.cs)

• Added Clear() method to reset buffer state (used for background/foreground transitions)
• Added SkipRead() method for advanced buffer control

RtcAudioSource (RtcAudioSource.cs)

• Fixed async audio capture safety: Each captured frame now gets its own NativeArray buffer
• Why: Native encoder consumes frames asynchronously after request.Send() returns; reusing a single buffer caused race conditions
• Track pending frames with unique async IDs and release buffers only after callback completes
• Added comprehensive logging for debugging audio capture pipeline
• Validation for sample rate/channel mismatches and malformed frames

AudioResampler & FFIClient

• Improved memory management and disposal patterns
• Better error handling and logging

iOS Build Support (IosLinkOrderDiagnostics.cs)

• New post-build processor to fix iOS link order issues
• Ensures liblivekit_ffi.a links before libiPhone-lib.a to prevent Opus/CELT conflicts
• Strips conflicting CELT object files from libiPhone-lib.a