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>

tests/test_m1_integration.cpp

@@ -42,6 +42,10 @@ struct EventStore {
     std::string last_error;
     bool disconnected{false};
     vc_connection_state last_state{VC_STATE_DISCONNECTED};
+
+    // Set right after vc_client_create, before vc_connect — lets on_event auto-confirm the
+    // M4 TOFU gate (VC_EVENT_SERVER_IDENTITY below) for this headless test.
+    vc_client* client{nullptr};
 };
 
 static void on_event(void* user, const vc_event* ev) {
@@ -49,6 +53,10 @@ static void on_event(void* user, const vc_event* ev) {
     std::lock_guard lk(s->mu);
     s->last_state = ev->connection_state;
     switch (ev->type) {
+        case VC_EVENT_SERVER_IDENTITY:
+            // No human to ask in a headless test — trust on first connect unconditionally.
+            vc_confirm_server_identity(s->client, 1);
+            break;
         case VC_EVENT_AUTH_RESULT:
             s->auth_result = static_cast<vc_result>(ev->result);
             s->auth_ok     = (ev->result == VC_OK);
@@ -171,6 +179,7 @@ int main() {
     vc_config cfgA{"test-clientA", "0.1", VC_LOG_OFF};
     vc_client* clientA = vc_client_create(&cfgA, cbA);
     CHECK(clientA != nullptr);
+    evA.client = clientA;
 
     CHECK(vc_connect(clientA, "127.0.0.1", port) == VC_OK);
     CHECK(vc_authenticate_guest(clientA, "GuestBob") == VC_OK);
@@ -190,6 +199,7 @@ int main() {
     vc_config cfgB{"test-clientB", "0.1", VC_LOG_OFF};
     vc_client* clientB = vc_client_create(&cfgB, cbB);
     CHECK(clientB != nullptr);
+    evB.client = clientB;
 
     CHECK(vc_connect(clientB, "127.0.0.1", port) == VC_OK);
     CHECK(vc_authenticate_user(clientB, "alice", "test-pass-alice") == VC_OK);