Multi-mic selection with audio mixer integration

[Copilot]

The app only supports one mic at a time. This adds an audio mixer that mixes one or more mics into a single output stream, and updates the UI to allow multi-mic selection.

Audio mixer integration (AudioUnit.swift)

• Integrates the existing (unused) AudioMixer class into the audio pipeline
• When >1 source is registered, lazily starts the mixer with a periodic timer rendering mixed output to the encoder
• Converts between CMSampleBuffer and AVAudioPCMBuffer at the mixer boundary
• Falls back to the existing direct single-source path when ≤1 source is active — zero behavior change for single-mic users
• Exposes addMixerSource/removeMixerSource/addMixerBuiltinSource/removeMixerBuiltinSource through Processor and Media

Multi-mic selection model (ModelAudio.swift)

• Mic class gains selectedMicIds: Set<String> and isSelected(mic:)
• manualToggleMicById toggles mics in/out of the mixer via addMicToMixer/removeMicFromMixer helpers that dispatch to the correct mixer API per mic type (builtin, RTMP, SRTLA, RIST, media player)
• Existing selectMic (auto-switch, scene override) resets to single-mic mode for backward compat

UI changes

• QuickButtonMicView: Single-select checkmark → multi-select toggle (circle / checkmark.circle.fill), calls manualToggleMicById. Removed swipe-to-delete.
• AudioSettingsView / StreamOverlayLeftView: Show comma-separated names of all selected mics

// Usage from app code
media.addMixerBuiltinSource()           // built-in mic
media.addMixerSource(sourceId: rtmpId)  // network camera mic
media.removeMixerSource(sourceId: rtmpId)

Tests (AudioMixerSuite.swift)

• Added three-input mixing test and dynamic add/remove test

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