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.App/Program.cs

@@ -0,0 +1,36 @@
+using VoiceCat.App.Forms;
+
+namespace VoiceCat.App;
+
+internal static class Program
+{
+    [STAThread]
+    private static void Main()
+    {
+        // Diagnostic-logging-only for now (manual debugging session) — every exception that
+        // would otherwise be silently caught by WinForms' default message-loop handling (or
+        // crash with no visible cause) gets printed to stdout/stderr first.
+        Application.ThreadException += (_, e) =>
+            Console.Error.WriteLine($"[UNHANDLED ThreadException] {e.Exception}");
+        AppDomain.CurrentDomain.UnhandledException += (_, e) =>
+            Console.Error.WriteLine($"[UNHANDLED AppDomain exception] {e.ExceptionObject}");
+
+        Console.WriteLine("VoiceCat.App starting...");
+        ApplicationConfiguration.Initialize();
+
+        using var connectDialog = new ConnectDialog();
+        Console.WriteLine("Showing ConnectDialog...");
+        var result = connectDialog.ShowDialog();
+        Console.WriteLine($"ConnectDialog closed with DialogResult={result}, ConnectedClient={(connectDialog.ConnectedClient is null ? "null" : "set")}");
+        if (result != DialogResult.OK || connectDialog.ConnectedClient is null)
+        {
+            Console.WriteLine("Exiting (cancelled or no connected client).");
+            return;
+        }
+
+        Console.WriteLine("Launching MainForm...");
+        Application.Run(new MainForm(connectDialog.ConnectedClient, connectDialog.SelfUserId,
+            connectDialog.Nickname));
+        Console.WriteLine("MainForm closed. Exiting.");
+    }
+}