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>

CMakePresets.json

@@ -61,13 +61,31 @@
         "VOICECAT_BUILD_TOOLS": "ON",
         "VCPKG_TARGET_TRIPLET": "x64-mingw-static"
       }
+    },
+    {
+      "name": "windows-client",
+      "inherits": "vcpkg-base",
+      "displayName": "Windows client (shared libvoicecat.dll for the C# WinForms app, M4)",
+      "description": "Produces a redistributable Release voicecat.dll with no MinGW runtime DLL dependencies (see core/CMakeLists.txt's static-runtime link flags and clients/windows/README.md). Tools/tests are off — this preset exists only to build the DLL.",
+      "binaryDir": "${sourceDir}/build/windows-client",
+      "cacheVariables": {
+        "CMAKE_BUILD_TYPE": "Release",
+        "VOICECAT_USE_VCPKG_DEPS": "ON",
+        "VOICECAT_BUILD_SHARED": "ON",
+        "VOICECAT_BUILD_SERVER": "OFF",
+        "VOICECAT_BUILD_TOOLS": "OFF",
+        "VOICECAT_BUILD_TESTS": "OFF",
+        "VCPKG_TARGET_TRIPLET": "x64-mingw-static",
+        "VCPKG_HOST_TRIPLET": "x64-mingw-static"
+      }
     }
   ],
   "buildPresets": [
-    { "name": "dev",            "configurePreset": "dev" },
-    { "name": "m1-dev",         "configurePreset": "m1-dev" },
-    { "name": "m2-dev",         "configurePreset": "m2-dev" },
-    { "name": "server-release", "configurePreset": "server-release" }
+    { "name": "dev",             "configurePreset": "dev" },
+    { "name": "m1-dev",          "configurePreset": "m1-dev" },
+    { "name": "m2-dev",          "configurePreset": "m2-dev" },
+    { "name": "server-release",  "configurePreset": "server-release" },
+    { "name": "windows-client",  "configurePreset": "windows-client" }
   ],
   "testPresets": [
     { "name": "dev",    "configurePreset": "dev",    "output": { "outputOnFailure": true } },