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_vad_ptt_devices.cpp

@@ -62,12 +62,20 @@ struct EventStore {
     bool     disconnected{false};
 
     const char* label{nullptr};
+
+    // 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) {
     auto* s = static_cast<EventStore*>(user);
     std::lock_guard lk(s->mu);
     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_ok      = (ev->result == VC_OK);
             s->self_user_id = ev->user_id;
@@ -273,6 +281,7 @@ static void test_vad_and_ptt_gate() {
     vc_config cfgA{"test-A", "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, "VP-A") == VC_OK);
     CHECK(wait_for(evA, [](EventStore& s) { return s.auth_ok; }, 8000));
@@ -284,6 +293,7 @@ static void test_vad_and_ptt_gate() {
     vc_config cfgB{"test-B", "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_guest(clientB, "VP-B") == VC_OK);
     CHECK(wait_for(evB, [](EventStore& s) { return s.auth_ok; }, 8000));