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.

CLAUDE.md

@@ -10,7 +10,8 @@ and what's next* read [`PROGRESS.md`](PROGRESS.md); for *design* read [`docs/`](
 > at `clients/apple/macOS/`. **iOS SwiftUI client shipped** — `VoiceCatiOS.xcodeproj` at
 > `clients/apple/iOS/`. `ctest --preset dev` green — 23/23 tests.
 > External PCM feed/tap API (`vc_stream_feed_pcm` + `vc_set_pcm_sink`) shipped.
-> Next: ReplayKit Broadcast Extension (iOS) or ScreenCaptureKit (macOS). See [`PROGRESS.md`](PROGRESS.md).
+> **Screen-audio sharing shipped on macOS (ScreenCaptureKit) and iOS (ReplayKit Broadcast
+> Upload Extension → host App Group ring → `vc_stream_feed_pcm`).** See [`PROGRESS.md`](PROGRESS.md).
 
 VoiceCat = self-hosted native voice & text chat (TeamSpeak/Mumble-style). Plain TCP (control)
 + UDP (media), no WebRTC, encrypted by default. A shared C++ core (`libvoicecat`) drives