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/CMakeLists.txt

@@ -95,4 +95,22 @@ if(VOICECAT_USE_VCPKG_DEPS)
     target_include_directories(test_vad_ptt_devices PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
     add_test(NAME vad_ptt_devices COMMAND test_vad_ptt_devices)
     set_tests_properties(vad_ptt_devices PROPERTIES TIMEOUT 60)
+
+    # M4: channel/user/stream snapshot getters (vc_list_channels/vc_list_users/
+    # vc_list_user_streams) — through the real C ABI against a real in-process server.
+    add_executable(test_channel_user_list_abi test_channel_user_list_abi.cpp)
+    target_link_libraries(test_channel_user_list_abi PRIVATE voicecat::server)
+    target_compile_features(test_channel_user_list_abi PRIVATE cxx_std_20)
+    target_include_directories(test_channel_user_list_abi PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
+    add_test(NAME channel_user_list_abi COMMAND test_channel_user_list_abi)
+    set_tests_properties(channel_user_list_abi PROPERTIES TIMEOUT 60)
+
+    # M4: TOFU server-identity gate (VC_EVENT_SERVER_IDENTITY / vc_confirm_server_identity /
+    # vc_get_server_identity_display) — real TLS handshakes against real in-process servers.
+    add_executable(test_tofu_flow test_tofu_flow.cpp)
+    target_link_libraries(test_tofu_flow PRIVATE voicecat::server)
+    target_compile_features(test_tofu_flow PRIVATE cxx_std_20)
+    target_include_directories(test_tofu_flow PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
+    add_test(NAME tofu_flow COMMAND test_tofu_flow)
+    set_tests_properties(tofu_flow PROPERTIES TIMEOUT 90)
 endif()