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>

core/src/voicecat.cpp

@@ -143,7 +143,19 @@ vc_result vc_get_stream_audio_config(vc_client* c, uint32_t user_id, uint32_t st
 vc_result vc_test_inject_capture(vc_client* c, uint32_t stream_id, const int16_t* pcm,
                                  size_t samples) {
     if (c == nullptr || pcm == nullptr) return VC_ERR_INVALID_ARG;
-    return c->test_inject_capture(stream_id, pcm, samples);
+    return c->stream_feed_pcm(stream_id, pcm, samples, 1);
+}
+
+vc_result vc_stream_feed_pcm(vc_client* c, uint32_t stream_id, const int16_t* pcm,
+                              size_t samples_per_channel, uint32_t channels) {
+    if (c == nullptr || pcm == nullptr) return VC_ERR_INVALID_ARG;
+    if (channels != 1 && channels != 2) return VC_ERR_INVALID_ARG;
+    return c->stream_feed_pcm(stream_id, pcm, samples_per_channel, channels);
+}
+
+vc_result vc_set_pcm_sink(vc_client* c, vc_pcm_sink_cb cb, void* user) {
+    if (c == nullptr) return VC_ERR_INVALID_ARG;
+    return c->set_pcm_sink(cb, user);
 }
 
 vc_result vc_set_capture_channels(vc_client* c, uint32_t stream_id, uint32_t channels) {