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/session/session.cpp

@@ -55,8 +55,11 @@ void SessionModel::apply_snapshot(const voicecat::v1::ServerStateSnapshot& snap)
     channels_.clear();
     for (const auto& pb : snap.channels()) {
         Channel ch;
-        ch.id   = pb.id();
-        ch.name = pb.name();
+        ch.id                 = pb.id();
+        ch.parent_id          = pb.parent_id();
+        ch.name               = pb.name();
+        ch.password_protected = pb.password_protected();
+        ch.max_users          = pb.max_users();
         channels_.push_back(std::move(ch));
     }
 
@@ -103,8 +106,11 @@ void SessionModel::apply_channel_event(const voicecat::v1::ChannelEvent& ev) {
     if (ev.kind() == Kind::CREATED || ev.kind() == Kind::UPDATED) {
         const auto& pb = ev.channel();
         Channel ch;
-        ch.id   = pb.id();
-        ch.name = pb.name();
+        ch.id                 = pb.id();
+        ch.parent_id          = pb.parent_id();
+        ch.name               = pb.name();
+        ch.password_protected = pb.password_protected();
+        ch.max_users          = pb.max_users();
 
         auto it = std::find_if(channels_.begin(), channels_.end(),
                                [&](const Channel& x) { return x.id == ch.id; });
@@ -112,7 +118,11 @@ void SessionModel::apply_channel_event(const voicecat::v1::ChannelEvent& ev) {
         else                       channels_.push_back(std::move(ch));
 
     } else if (ev.kind() == Kind::DELETED) {
-        uint32_t cid = ev.channel().id();
+        // deleted_id, not channel().id() — the proto leaves `channel` unset for deletes
+        // (docs/protocol.md, core/proto/voicecat.proto's ChannelEvent). Pre-existing bug, dead
+        // code until something actually emits ChannelEvent (no channel CRUD exists yet — M5+),
+        // fixed here while touching this function for the M4 field-population fix.
+        uint32_t cid = ev.deleted_id();
         channels_.erase(std::remove_if(channels_.begin(), channels_.end(),
                                        [cid](const Channel& x) { return x.id == cid; }),
                         channels_.end());