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/net/transport.cpp

@@ -324,11 +324,34 @@ void TcpServerConn::close() {
 
 // ── TcpAcceptor ─────────────────────────────────────────────────────────────
 
-TcpAcceptor::TcpAcceptor(asio::io_context& io, uint16_t port, ConnFactory factory)
-    : acceptor_(io, asio::ip::tcp::endpoint(asio::ip::tcp::v4(), port)),
-      factory_(std::move(factory)) {
-    acceptor_.set_option(asio::ip::tcp::acceptor::reuse_address(true));
+namespace {
+// Try IPv6 dual-stack first (one socket handles both ::1 and 127.0.0.1 — fixes the common
+// Windows case where `localhost` resolves to ::1 before 127.0.0.1). Falls back to IPv4-only
+// if the OS has IPv6 disabled or the dual-stack bind fails for any reason.
+asio::ip::tcp::acceptor make_acceptor(asio::io_context& io, uint16_t port) {
+    asio::ip::tcp::acceptor acc(io);
+    std::error_code ec;
+    acc.open(asio::ip::tcp::v6(), ec);
+    if (!ec) {
+        acc.set_option(asio::ip::v6_only(false), ec);  // dual-stack
+        acc.set_option(asio::ip::tcp::acceptor::reuse_address(true));
+        acc.bind(asio::ip::tcp::endpoint(asio::ip::tcp::v6(), port), ec);
+        if (!ec) acc.listen(asio::socket_base::max_listen_connections, ec);
+    }
+    if (ec) {
+        if (acc.is_open()) { std::error_code ignored; acc.close(ignored); }
+        acc.open(asio::ip::tcp::v4());
+        acc.set_option(asio::ip::tcp::acceptor::reuse_address(true));
+        acc.bind(asio::ip::tcp::endpoint(asio::ip::tcp::v4(), port));
+        acc.listen(asio::socket_base::max_listen_connections);
+    }
+    return acc;
 }
+}  // namespace
+
+TcpAcceptor::TcpAcceptor(asio::io_context& io, uint16_t port, ConnFactory factory)
+    : acceptor_(make_acceptor(io, port)),
+      factory_(std::move(factory)) {}
 
 void TcpAcceptor::start() { do_accept(); }