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/protocol.md

@@ -309,7 +309,21 @@ message TextMessage {
   disconnect; server-sent fatal `Disconnect` uses `code ≥ 1` (1 = protocol error,
   2 = kicked).
 
-## 8. Extensibility checklist
+## 8. Client-local features (no protocol changes)
+
+Some features are entirely client-side and involve no changes to the wire format:
+
+- **External PCM feed (`vc_stream_feed_pcm`)** — the caller supplies interleaved int16 PCM
+  that the core frames, encodes, and sends over the existing UDP media path. From the server
+  and peers' perspective the stream is indistinguishable from a hardware-captured stream. No
+  new messages, fields, or tags are needed.
+- **PCM tap (`vc_set_pcm_sink`)** — receives decoded per-stream audio before hardware mixing.
+  Entirely local to the listener; no protocol traffic of any kind.
+
+These are noted here to prevent future contributors from looking for corresponding protocol
+changes: there are none.
+
+## 9. Extensibility checklist
 
 When adding a feature later (e.g. **file transfer**), the rules are: