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/Structs.cs

@@ -0,0 +1,127 @@
+using System.Runtime.InteropServices;
+
+// Native (blittable) struct layouts mirroring voicecat.h field-for-field. These are the raw
+// P/Invoke shapes — NativeMethods.cs uses them directly; Marshaling.cs converts them to the
+// managed record types in Models.cs. const char* fields stay IntPtr here (LibraryImport's
+// StringMarshalling only auto-converts top-level string parameters/returns, not struct
+// fields) and must be hand-marshaled — see Marshaling.cs and VoiceCatClient.cs.
+namespace VoiceCat.Interop;
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcEventNative
+{
+    public VcEventType Type;
+    public VcConnectionState ConnectionState;
+    public int Result; // vc_result
+    public uint UserId;
+    public uint ChannelId;
+    public uint StreamId;
+    public VcTextScope TextScope;
+    public uint U32a;
+    public IntPtr Text; // owned by core, valid ONLY for the callback's duration
+    public ulong TimestampUnixMs;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcConfigNative
+{
+    public IntPtr ClientName;
+    public IntPtr ClientVersion;
+    public VcLogLevel LogLevel;
+    public IntPtr TofuStorePath;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcCallbacksNative
+{
+    public IntPtr OnEvent;  // delegate* unmanaged<IntPtr, VcEventNative*, void>
+    public IntPtr OnLevel;  // delegate* unmanaged<IntPtr, uint, float, void>
+    public IntPtr User;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcStreamDescNative
+{
+    public VcStreamKind Kind;
+    public IntPtr DeviceId; // unused by vc_stream_start today — device selection is a
+                             // separate vc_set_input_device call; always IntPtr.Zero here.
+    public IntPtr Label;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcAudioConfigNative
+{
+    public uint Codec;
+    public uint Mode;
+    public uint SampleRate;
+    public uint BitrateBps;
+    public uint FrameMs;
+    public uint Application;
+    public int Fec;
+    public uint ExpectedPacketLoss;
+    public int Dtx;
+    public uint Complexity;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcDeviceNative
+{
+    public IntPtr Id;
+    public IntPtr Name;
+    public int IsDefault;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcDeviceListNative
+{
+    public IntPtr Items;
+    public nuint Count;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcChannelNative
+{
+    public uint Id;
+    public uint ParentId;
+    public IntPtr Name;
+    public int PasswordProtected;
+    public uint MaxUsers;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcChannelListNative
+{
+    public IntPtr Items;
+    public nuint Count;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcUserNative
+{
+    public uint Id;
+    public IntPtr Nickname;
+    public int IsGuest;
+    public uint ChannelId;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcUserListNative
+{
+    public IntPtr Items;
+    public nuint Count;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcStreamSummaryNative
+{
+    public uint StreamId;
+    public VcStreamKind Kind;
+    public IntPtr Label;
+}
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct VcStreamSummaryListNative
+{
+    public IntPtr Items;
+    public nuint Count;
+}