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>

clients/windows/VoiceCat.Interop/NativeMethods.cs

@@ -98,6 +98,25 @@ internal static partial class NativeMethods
     internal static unsafe partial VcResult vc_test_inject_capture(nint c, uint streamId,
         short* pcm, nuint samples);
 
+    // ── External PCM feed / tap ──────────────────────────────────────────────────────────
+
+    [LibraryImport(LibName)]
+    internal static unsafe partial VcResult vc_stream_feed_pcm(nint c, uint streamId,
+        short* pcm, nuint samplesPerChannel, uint channels);
+
+    // Delegate type for the PCM sink callback — callers convert to a native function
+    // pointer via Marshal.GetFunctionPointerForDelegate (for instance members) or by casting
+    // a static lambda to delegate* unmanaged<> (for [UnmanagedCallersOnly] statics).
+    // Keep the delegate alive for the lifetime of the sink registration.
+    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
+    public delegate void VcPcmSinkCallback(IntPtr user, uint userId, uint streamId,
+        IntPtr pcm, nuint samplesPerChannel, uint channels, uint sampleRate);
+
+    // cb is a raw function pointer (IntPtr.Zero = disable). Use
+    // Marshal.GetFunctionPointerForDelegate(sinkDelegate) to convert from VcPcmSinkCallback.
+    [LibraryImport(LibName)]
+    internal static partial VcResult vc_set_pcm_sink(nint c, nint cb, IntPtr user);
+
     // ── Text ─────────────────────────────────────────────────────────────────────────────
     [LibraryImport(LibName, StringMarshalling = StringMarshalling.Utf8)]
     internal static partial VcResult vc_send_text(nint c, VcTextScope scope, uint targetId,