feat(M4): Windows WinForms client, TOFU identity pinning, VAD threshold + always-on mode

Core ABI extensions (voicecat.h):
- vc_list_channels / vc_list_users / vc_list_user_streams — pull-based snapshot getters
  for the channel-tree and user-list UI; session_model_mu_ guards cross-thread reads
- VC_EVENT_JOIN_RESULT / vc_join_channel — channel join with optional password
- VC_EVENT_SERVER_IDENTITY + vc_confirm_server_identity — TOFU gate that blocks io_thread_
  until the UI approves or rejects; pins TLS leaf-cert SHA-256 (not declared Ed25519)
- vc_get_server_identity_display — Ed25519 fingerprint for human-readable display only
- VC_INPUT_ALWAYS_ON = 2 in vc_input_mode — transmit unconditionally, no VAD gate
- vc_set_vad_threshold — live RMS threshold update (0.0–1.0); EnergyVadProcessor stores
  it atomically so the audio RT path reads without a lock

C++ implementation:
- SessionModel::apply_snapshot / apply_channel_event fixed to populate parent_id,
  password_protected, and max_users (were permanently zeroed)
- TlsContext::peer_cert_fingerprint — SHA-256 of peer leaf cert DER via mbedTLS
- TofuStore split into peek (read-only) + pin (write) so first-connect only persists
  after user approval; tofu_store_path in vc_config for per-user pin file location
- TcpAcceptor uses dual-stack IPv6+IPv4 fallback (fixes localhost → ::1 on Windows)
- windows-client CMake preset: Release shared DLL, static MinGW runtime, no tools/tests
- New C++ tests: test_channel_user_list_abi, test_tofu_flow (14/14 green)

Windows client (clients/windows/ — .NET 10 WinForms):
- VoiceCat.Interop: LibraryImport P/Invoke surface, UnmanagedCallersOnly callbacks,
  Channel<VoiceCatEvent> event delivery drained by 30ms WinForms Timer
- VoiceCat.App: ConnectDialog (saved servers, DPAPI password storage), ServerIdentity-
  Dialog (TOFU first-connect / mismatch warning), MainForm (channel TreeView, user
  ListBox, RichTextBox chat, voice controls, device pickers, VAD/PTT/always-on mode,
  per-user gain/mute/NR tuning, VAD sensitivity TrackBar, level meter ProgressBar)
- PttKeyCaptureDialog — focus-scoped PTT key capture (documented limitation)
- PerUserTuningDialog — real-time gain/mute/NR applied to all of a user's streams
- Accessibility: explicit AccessibleName/Description on every control, & mnemonics,
  Activity log ListBox as durable screen-reader record, AutomationNotification for
  curated live announcements

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

clients/windows/VoiceCat.Interop/NativeCallbacks.cs

@@ -0,0 +1,33 @@
+using System.Runtime.InteropServices;
+
+// [UnmanagedCallersOnly] static methods for vc_callbacks.on_event/on_level — true native
+// function pointers, not GC-tracked delegates (avoids the classic P/Invoke pitfall where a
+// delegate is collected by the GC sometime after the call that registered it returns; see
+// docs/tech-stack.md §2). vc_callbacks.user is a GCHandle-wrapped VoiceCatClient (allocated in
+// VoiceCatClient's constructor, freed in Dispose) — these methods must be static, so they
+// resolve back to the right client instance via that handle rather than closing over state.
+namespace VoiceCat.Interop;
+
+internal static unsafe class NativeCallbacks
+{
+    [UnmanagedCallersOnly]
+    internal static void OnEvent(IntPtr userContext, VcEventNative* ev)
+    {
+        if (userContext == IntPtr.Zero) return;
+        if (GCHandle.FromIntPtr(userContext).Target is not VoiceCatClient client) return;
+
+        // CRITICAL (voicecat.h's vc_event doc comment): ev->Text is owned by the core and
+        // valid ONLY for the duration of this callback. Convert to a managed string NOW,
+        // before returning — never store/queue the raw VcEventNative across the callback
+        // boundary, or Text will be a dangling pointer by the time it's read.
+        client.EnqueueEvent(VoiceCatEvent.FromNative(*ev));
+    }
+
+    [UnmanagedCallersOnly]
+    internal static void OnLevel(IntPtr userContext, uint streamId, float rms)
+    {
+        if (userContext == IntPtr.Zero) return;
+        if (GCHandle.FromIntPtr(userContext).Target is not VoiceCatClient client) return;
+        client.EnqueueLevel(streamId, rms);
+    }
+}