feat: external PCM feed/tap API (vc_stream_feed_pcm + vc_set_pcm_sink)

Promotes vc_test_inject_capture (mono-only, TEST-ONLY) to a public,
stereo-capable production API and adds a symmetric PCM tap on the
receive side. Enables ReplayKit (iOS), ScreenCaptureKit (macOS), bots,
soundboards, and custom clients — all without a hardware audio device.

Core C++:
- voicecat.h: new vc_stream_feed_pcm, vc_pcm_sink_cb typedef,
  vc_set_pcm_sink; vc_test_inject_capture kept as deprecated alias
- audio_engine: stereo-aware inject_capture (channels param + ring
  reset on channel-count change); atomic pcm_sink_ fired per decoded
  frame in on_playback; RemoteStream carries user_id/stream_id for
  RT-safe sink metadata; init_recv_stream takes user_id+stream_id
- client.cpp: stream_feed_pcm / set_pcm_sink implementations;
  sync_remote_streams passes user_id/stream_id to init_recv_stream
- voicecat.cpp: trampolines + channels=1/2 validation

Tests: test_external_pcm (headless, 3 sub-tests: mono round-trip,
stereo feed L≠R, sink metadata+disable). ctest 23/23.

Swift: feedPcm / setPcmSink in VoiceCatClient.swift + 4 XCTest
smoke tests (ExternalPcmTests.swift).

C#: StreamFeedPcm / SetPcmSink in VoiceCatClient.cs + NativeMethods.cs
(vc_stream_feed_pcm unsafe P/Invoke, VcPcmSinkCallback delegate,
vc_set_pcm_sink via nint) + 4 xUnit smoke tests (ExternalPcmTests.cs).

Docs: architecture.md §4 new subsection, voice.md §9 updated
(macOS/iOS now reference vc_stream_feed_pcm), protocol.md §8 explicit
no-protocol-change note, roadmap.md M5 entry.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/roadmap.md

@@ -69,7 +69,8 @@ exists from M1 so the protocol can be exercised long before any GUI.
 - ~~AVAudioSession, mic permission, foreground voice.~~ ✓ Done — `IOSAudioRouter` drives
   all iOS audio routing (input ports, orientation/polar patterns, HFP/A2DP, Standard/Raw
   mic mode, stereo capture), `vc_audio_suspend`/`vc_audio_resume` for interruptions.
-- ReplayKit broadcast extension for `SCREEN_AUDIO`.
+- ReplayKit broadcast extension for `SCREEN_AUDIO` — feeds `CMSampleBuffer` audio via
+  `vc_stream_feed_pcm` (see architecture.md §4).
 
 **Exit:** non-technical user installs a client, saves a server, and joins.
 
@@ -78,6 +79,13 @@ exists from M1 so the protocol can be exercised long before any GUI.
   (channel CRUD with full Opus config, user moderation, server account management); macOS/iOS
   Swift UI pending.
 - DRED toggle, audio-quality polish. (AEC and VAD/PTT already shipped in M2.)
+- **External PCM feed/tap API** (`vc_stream_feed_pcm` + `vc_set_pcm_sink`) ✓ shipped
+  (2026-06-20) — promotes `vc_test_inject_capture` to a public, stereo-capable API and adds
+  a symmetric PCM sink. Enables ReplayKit (iOS), ScreenCaptureKit (macOS), bots, and custom
+  clients. See architecture.md §4 "External PCM feed/tap" and protocol.md §8 for the
+  no-protocol-change rationale. One new C++ ctest binary (`test_external_pcm`) covering 3
+  sub-tests (`test_feed_pcm_round_trip`, `test_feed_pcm_stereo`, `test_pcm_sink`) — ctest
+  23/23; Swift wrapper + 4 XCTest smoke tests; C# wrapper + 4 xUnit smoke tests.
 - **Then (post-v1, protocol already reserves space):** file transfer, E2EE option,
   CallKit/PushKit background voice, key-based identity, server-side text history,
   multi-node server.