feat(apple): screen-audio sharing -- macOS ScreenCaptureKit, iOS ReplayKit

Implement system/desktop audio sharing on the Apple clients, feeding the
existing SCREEN_AUDIO Opus -> AEAD -> UDP path via vc_stream_feed_pcm. No
C++/protocol/codec changes -- the core was already ready (the Windows-only
loopback is #ifdef VOICECAT_HAS_LOOPBACK; off Windows the stream just waits
for fed PCM). Audio only; video is dropped.

macOS (in-process):
- ScreenAudioCapture.swift drives an audio-only SCStream
  (excludesCurrentProcessAudio), converts Float32 -> int16 in the channel's
  mono/stereo mode, and calls feedPcm. Capture starts on the self
  .streamStarted event (effective config known then). Wired into
  MainWindowController.screenAudioClicked().

iOS (forward-to-host, single session):
- VoiceCatBroadcast: a ReplayKit Broadcast Upload Extension consumes
  .audioApp only, resamples to 48kHz int16 stereo (AVAudioConverter), and
  writes a shared App Group SPSC ring (BroadcastAudioRing.swift). It does
  not link libvoicecat.
- Host BroadcastAudioPump drains the ring (reacting to the extension's
  Darwin notifications) and feeds the SCREEN_AUDIO stream it owns, downmixing
  to mono when the channel is mono. Screen audio appears as a second stream
  of the same user; no credentials persisted. UI is RPSystemBroadcastPicker
  View in VoiceControlsView. Removes the speculative BroadcastCredentials.

Docs: voice.md s9, CLAUDE.md status, PROGRESS.md.

clients/apple/macOS/VoiceCatMac/Audio/ScreenAudioCapture.swift

@@ -0,0 +1,158 @@
+import AVFoundation
+import ScreenCaptureKit
+
+// ScreenAudioCapture — macOS system/desktop audio capture for the SCREEN_AUDIO stream.
+//
+// The macOS analog of the Windows WASAPI loopback path (docs/voice.md §9). ScreenCaptureKit
+// (macOS 13+) captures whatever the system is playing; we convert each audio CMSampleBuffer
+// (Float32) → int16 interleaved and push 20 ms frames (960 samples/channel @ 48 kHz) into the
+// core via `vc_stream_feed_pcm` (exposed as `VoiceCatClient.feedPcm`). The core then runs the
+// same Opus-encode → media-AEAD → UDP path as any other stream — only the *source* is
+// platform-specific (architecture.md §4).
+//
+// Audio-only: we request a 2×2 video plane at 1 fps purely because SCStream needs a video
+// configuration, and we never add a `.screen` output — only `.audio`. `excludesCurrentProcess
+// Audio` prevents the self-echo loop of re-capturing our own incoming voice mix.
+//
+// `feedPcm` is thread-safe (any thread), so we forward straight from the sample-handler queue.
+final class ScreenAudioCapture: NSObject, SCStreamOutput, SCStreamDelegate {
+
+    /// Receives a full 20 ms frame: (interleaved int16 PCM, samplesPerChannel = 960, channels).
+    typealias PcmHandler = (UnsafePointer<Int16>, Int, UInt32) -> Void
+
+    enum CaptureError: Error { case noDisplay }
+
+    private static let frameSamplesPerChannel = 960  // 20 ms @ 48 kHz
+
+    private let onPcm: PcmHandler
+    private let channels: Int  // 1 (mono) or 2 (stereo interleaved), matches the stream's mode
+    private let sampleQueue = DispatchQueue(label: "cat.voice.screenaudio.samples")
+    private var stream: SCStream?
+
+    /// Interleaved int16 carry-over between callbacks (ScreenCaptureKit buffers don't align to
+    /// 20 ms), drained in whole `frameSamplesPerChannel * channels` chunks. Only touched on
+    /// `sampleQueue`.
+    private var pending: [Int16] = []
+
+    init(channels: UInt32, onPcm: @escaping PcmHandler) {
+        self.channels = max(1, min(2, Int(channels)))
+        self.onPcm = onPcm
+        super.init()
+    }
+
+    /// Begin capture. Throws if Screen Recording permission is denied (the first
+    /// `SCShareableContent.current` access is what surfaces the TCC prompt) or no display exists.
+    func start() async throws {
+        let content = try await SCShareableContent.current
+        guard let display = content.displays.first else { throw CaptureError.noDisplay }
+
+        let filter = SCContentFilter(display: display, excludingWindows: [])
+
+        let config = SCStreamConfiguration()
+        config.capturesAudio = true
+        config.excludesCurrentProcessAudio = true
+        config.sampleRate = 48000
+        config.channelCount = channels
+        // SCStream requires a video config even when we only consume audio — keep it minimal.
+        config.width = 2
+        config.height = 2
+        config.minimumFrameInterval = CMTime(value: 1, timescale: 1)  // ~1 fps
+        config.queueDepth = 6
+
+        let stream = SCStream(filter: filter, configuration: config, delegate: self)
+        try stream.addStreamOutput(self, type: .audio, sampleHandlerQueue: sampleQueue)
+        try await stream.startCapture()
+        self.stream = stream
+    }
+
+    /// Stop capture and release the stream. Safe to call multiple times.
+    func stop() {
+        guard let stream else { return }
+        self.stream = nil
+        Task { try? await stream.stopCapture() }
+    }
+
+    // MARK: - SCStreamOutput
+
+    func stream(_ stream: SCStream, didOutputSampleBuffer sampleBuffer: CMSampleBuffer,
+                of type: SCStreamOutputType) {
+        guard type == .audio, CMSampleBufferDataIsReady(sampleBuffer) else { return }
+        guard let fmt = sampleBuffer.formatDescription,
+              let asbd = fmt.audioStreamBasicDescription else { return }
+        // ScreenCaptureKit always delivers Float32 PCM; bail on anything unexpected.
+        guard asbd.mFormatFlags & kAudioFormatFlagIsFloat != 0 else { return }
+
+        let nonInterleaved = asbd.mFormatFlags & kAudioFormatFlagIsNonInterleaved != 0
+        let srcChannels = max(1, Int(asbd.mChannelsPerFrame))
+
+        try? sampleBuffer.withAudioBufferList { ablPtr, _ in
+            convert(ablPtr, nonInterleaved: nonInterleaved, srcChannels: srcChannels)
+        }
+    }
+
+    // MARK: - Conversion (called on sampleQueue)
+
+    private func convert(_ abl: UnsafeMutableAudioBufferListPointer,
+                         nonInterleaved: Bool, srcChannels: Int) {
+        guard let first = abl.first, first.mData != nil else { return }
+
+        let out = channels
+        var interleaved: [Int16]
+
+        if nonInterleaved {
+            // One buffer per source channel; each is `frames` Float32 samples.
+            let frames = Int(first.mDataByteSize) / MemoryLayout<Float>.size
+            if frames == 0 { return }
+            let ch0 = first.mData!.assumingMemoryBound(to: Float.self)
+            let ch1: UnsafePointer<Float>? = (abl.count > 1)
+                ? UnsafePointer(abl[1].mData!.assumingMemoryBound(to: Float.self)) : nil
+            interleaved = [Int16](repeating: 0, count: frames * out)
+            for i in 0..<frames {
+                let l = ch0[i]
+                let r = ch1?[i] ?? l
+                if out == 2 {
+                    interleaved[i * 2]     = Self.f2i(l)
+                    interleaved[i * 2 + 1] = Self.f2i(srcChannels >= 2 ? r : l)
+                } else {
+                    interleaved[i] = Self.f2i(srcChannels >= 2 ? (l + r) * 0.5 : l)
+                }
+            }
+        } else {
+            // Single interleaved Float32 buffer, srcChannels wide.
+            let total = Int(first.mDataByteSize) / MemoryLayout<Float>.size
+            let frames = total / srcChannels
+            if frames == 0 { return }
+            let src = first.mData!.assumingMemoryBound(to: Float.self)
+            interleaved = [Int16](repeating: 0, count: frames * out)
+            for i in 0..<frames {
+                let l = src[i * srcChannels]
+                let r = srcChannels >= 2 ? src[i * srcChannels + 1] : l
+                if out == 2 {
+                    interleaved[i * 2]     = Self.f2i(l)
+                    interleaved[i * 2 + 1] = Self.f2i(r)
+                } else {
+                    interleaved[i] = Self.f2i(srcChannels >= 2 ? (l + r) * 0.5 : l)
+                }
+            }
+        }
+
+        emit(interleaved)
+    }
+
+    /// Accumulate interleaved int16 and fire `onPcm` for every whole 20 ms frame.
+    private func emit(_ interleaved: [Int16]) {
+        pending.append(contentsOf: interleaved)
+        let full = Self.frameSamplesPerChannel * channels
+        while pending.count >= full {
+            pending.withUnsafeBufferPointer { buf in
+                onPcm(buf.baseAddress!, Self.frameSamplesPerChannel, UInt32(channels))
+            }
+            pending.removeFirst(full)
+        }
+    }
+
+    private static func f2i(_ f: Float) -> Int16 {
+        let v = max(-1.0, min(1.0, f)) * 32767.0
+        return Int16(v.rounded())
+    }
+}