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>

tests/CMakeLists.txt

@@ -183,4 +183,15 @@ if(VOICECAT_USE_VCPKG_DEPS)
     target_include_directories(test_dred_toggle PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
     add_test(NAME dred_toggle COMMAND test_dred_toggle)
     set_tests_properties(dred_toggle PROPERTIES TIMEOUT 60)
+
+    # External PCM feed/tap API:
+    #   test_feed_pcm_round_trip — vc_stream_feed_pcm (mono), verified via pcm_sink.
+    #   test_feed_pcm_stereo     — vc_stream_feed_pcm (stereo, Music Room), L != R assertion.
+    #   test_pcm_sink            — vc_set_pcm_sink metadata (user_id / stream_id / sr) + disable.
+    add_executable(test_external_pcm test_external_pcm.cpp)
+    target_link_libraries(test_external_pcm PRIVATE voicecat::server)
+    target_compile_features(test_external_pcm PRIVATE cxx_std_20)
+    target_include_directories(test_external_pcm PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
+    add_test(NAME external_pcm COMMAND test_external_pcm)
+    set_tests_properties(external_pcm PROPERTIES TIMEOUT 90)
 endif()