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>

core/src/voicecat.cpp

@@ -102,6 +102,11 @@ vc_result vc_set_input_mode(vc_client* c, vc_input_mode mode) {
     return c->set_input_mode(mode);
 }
 
+vc_result vc_set_vad_threshold(vc_client* c, float threshold) {
+    if (c == nullptr) return VC_ERR_INVALID_ARG;
+    return c->set_vad_threshold(threshold);
+}
+
 vc_result vc_set_push_to_talk(vc_client* c, int active) {
     if (c == nullptr) return VC_ERR_INVALID_ARG;
     return c->set_push_to_talk(active != 0);
@@ -152,4 +157,54 @@ void vc_free_device_list(vc_device_list* list) {
     list->count = 0;
 }
 
+vc_result vc_list_channels(vc_client* c, vc_channel_list* out) {
+    if (c == nullptr || out == nullptr) return VC_ERR_INVALID_ARG;
+    return c->list_channels(out);
+}
+
+void vc_free_channel_list(vc_channel_list* list) {
+    if (list == nullptr || list->items == nullptr) return;
+    for (size_t i = 0; i < list->count; ++i) delete[] list->items[i].name;
+    delete[] list->items;
+    list->items = nullptr;
+    list->count = 0;
+}
+
+vc_result vc_list_users(vc_client* c, vc_user_list* out) {
+    if (c == nullptr || out == nullptr) return VC_ERR_INVALID_ARG;
+    return c->list_users(out);
+}
+
+void vc_free_user_list(vc_user_list* list) {
+    if (list == nullptr || list->items == nullptr) return;
+    for (size_t i = 0; i < list->count; ++i) delete[] list->items[i].nickname;
+    delete[] list->items;
+    list->items = nullptr;
+    list->count = 0;
+}
+
+vc_result vc_list_user_streams(vc_client* c, uint32_t user_id, vc_stream_summary_list* out) {
+    if (c == nullptr || out == nullptr) return VC_ERR_INVALID_ARG;
+    return c->list_user_streams(user_id, out);
+}
+
+void vc_free_stream_summary_list(vc_stream_summary_list* list) {
+    if (list == nullptr || list->items == nullptr) return;
+    for (size_t i = 0; i < list->count; ++i) delete[] list->items[i].label;
+    delete[] list->items;
+    list->items = nullptr;
+    list->count = 0;
+}
+
+vc_result vc_confirm_server_identity(vc_client* c, int accept) {
+    if (c == nullptr) return VC_ERR_INVALID_ARG;
+    return c->confirm_server_identity(accept != 0);
+}
+
+vc_result vc_get_server_identity_display(vc_client* c, char* out_buf, size_t buf_cap,
+                                         size_t* out_len) {
+    if (c == nullptr) return VC_ERR_INVALID_ARG;
+    return c->get_server_identity_display(out_buf, buf_cap, out_len);
+}
+
 }  // extern "C"