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/CMakeLists.txt

@@ -5,6 +5,24 @@ file(GLOB_RECURSE VOICECAT_SOURCES CONFIGURE_DEPENDS
 
 if(VOICECAT_BUILD_SHARED)
     add_library(voicecat SHARED ${VOICECAT_SOURCES})
+    if(WIN32 AND MINGW)
+        # M4: the C# client only ships voicecat.dll itself — no MinGW runtime DLLs alongside
+        # it. x64-mingw-static only statically links vcpkg's OWN library deps (protobuf,
+        # sodium, mbedTLS, ...); the GCC/MinGW runtime stays dynamic by default
+        # (libgcc_s_seh-1.dll/libwinpthread-1.dll/libstdc++-6.dll — confirmed via `objdump -p`
+        # on the existing vccli.exe). These flags are the standard fully-static-MinGW
+        # recipe. Verify after building (see clients/windows/README.md):
+        #   objdump -p build/windows-client/bin/voicecat.dll | grep "DLL Name"
+        # should show only Windows system DLLs.
+        target_link_options(voicecat PRIVATE
+            -static-libgcc -static-libstdc++ -static -lwinpthread)
+    endif()
+    if(WIN32)
+        # CMake's default SHARED naming on MinGW adds a "lib" prefix (libvoicecat.dll) —
+        # drop it so the output is exactly voicecat.dll, matching the C ABI/library name the
+        # C# [LibraryImport] surface and docs use everywhere else.
+        set_target_properties(voicecat PROPERTIES PREFIX "")
+    endif()
 else()
     add_library(voicecat STATIC ${VOICECAT_SOURCES})
     # Static consumers must see VC_API as empty (no dllimport).