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/VoiceCatClient.cs

@@ -225,6 +225,20 @@ public sealed class VoiceCatClient : IDisposable
         return (r, r == VcResult.Ok ? Marshaling.ToManaged(in native) : null);
     }
 
+    // ── External PCM feed / tap ─────────────────────────────────────────────────────────
+    public unsafe VcResult StreamFeedPcm(uint streamId, ReadOnlySpan<short> pcm,
+        int samplesPerChannel, uint channels)
+    {
+        fixed (short* p = pcm)
+            return NativeMethods.vc_stream_feed_pcm(_handle.DangerousGetHandle(),
+                streamId, p, (nuint)samplesPerChannel, channels);
+    }
+
+    // Pass Marshal.GetFunctionPointerForDelegate(cb) for a managed delegate, or
+    // IntPtr.Zero to disable. Keep the delegate alive for the lifetime of the registration.
+    public VcResult SetPcmSink(nint cb, IntPtr user) =>
+        NativeMethods.vc_set_pcm_sink(_handle.DangerousGetHandle(), cb, user);
+
     // ── M5: Moderation & admin ───────────────────────────────────────────────────────────
     public VcResult KickUser(uint userId, string? reason = null) =>
         NativeMethods.vc_kick_user(_handle.DangerousGetHandle(), userId, reason);