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>

clients/windows/VoiceCat.App/Forms/PerUserTuningDialog.cs

@@ -0,0 +1,125 @@
+using VoiceCat.Interop;
+
+namespace VoiceCat.App.Forms;
+
+/// <summary>
+/// Per-remote-user gain, mute, and noise reduction settings.
+/// Changes are applied in real-time as the user adjusts controls — no OK/Cancel round-trip
+/// for gain and mute (the close button just dismisses). The settings live in the core and
+/// are not persisted across sessions.
+/// </summary>
+public sealed class PerUserTuningDialog : Form
+{
+    private readonly VoiceCatClient _client;
+    private readonly uint _userId;
+
+    private readonly TrackBar _trkGain;
+    private readonly Label _lblGainValue;
+    private readonly CheckBox _chkMute;
+    private readonly CheckBox _chkNr;
+
+    public PerUserTuningDialog(VoiceCatClient client, uint userId, string nickname)
+    {
+        _client = client;
+        _userId = userId;
+
+        // ── Controls ──────────────────────────────────────────────────────────
+        var lblTitle = new Label
+        {
+            Text = $"Settings for {nickname}",
+            AutoSize = true,
+            Font = new Font(Font, FontStyle.Bold),
+            Location = new Point(12, 12),
+            TabIndex = 0,
+        };
+
+        var lblGainLabel = new Label
+        {
+            Text = "&Gain:",
+            AutoSize = true,
+            Location = new Point(12, 46),
+            TabIndex = 1,
+        };
+
+        _trkGain = new TrackBar
+        {
+            AccessibleName = "Gain",
+            AccessibleDescription = "Volume level for this user. 100 is normal (1.0×), 200 is double.",
+            Location = new Point(55, 38),
+            Size = new Size(220, 45),
+            Minimum = 0,
+            Maximum = 200,
+            Value = 100,
+            TickFrequency = 25,
+            SmallChange = 5,
+            LargeChange = 25,
+            TabIndex = 2,
+        };
+
+        _lblGainValue = new Label
+        {
+            Text = "100% (1.0×)",
+            AutoSize = true,
+            Location = new Point(280, 46),
+            TabIndex = 3,
+        };
+
+        _chkMute = new CheckBox
+        {
+            Text = "&Mute this user",
+            AutoSize = true,
+            Location = new Point(12, 92),
+            TabIndex = 4,
+        };
+
+        _chkNr = new CheckBox
+        {
+            Text = "&Noise reduction (planned — currently passthrough)",
+            AutoSize = true,
+            Location = new Point(12, 118),
+            TabIndex = 5,
+        };
+
+        var btnClose = new Button
+        {
+            Text = "&Close",
+            DialogResult = DialogResult.OK,
+            Location = new Point(296, 152),
+            Size = new Size(75, 27),
+            TabIndex = 6,
+        };
+
+        // ── Wire events ───────────────────────────────────────────────────────
+        _trkGain.Scroll += (_, _) =>
+        {
+            float gain = _trkGain.Value / 100f;
+            _lblGainValue.Text = $"{_trkGain.Value}% ({gain:F1}×)";
+            ApplySettings();
+        };
+        _chkMute.CheckedChanged += (_, _) => ApplySettings();
+        _chkNr.CheckedChanged += (_, _) => ApplySettings();
+
+        // ── Form ──────────────────────────────────────────────────────────────
+        AcceptButton = btnClose;
+        AutoScaleMode = AutoScaleMode.Font;
+        ClientSize = new Size(384, 192);
+        Controls.AddRange([lblTitle, lblGainLabel, _trkGain, _lblGainValue,
+                           _chkMute, _chkNr, btnClose]);
+        FormBorderStyle = FormBorderStyle.FixedDialog;
+        MaximizeBox = false;
+        MinimizeBox = false;
+        StartPosition = FormStartPosition.CenterParent;
+        Text = $"User settings — {nickname}";
+    }
+
+    private void ApplySettings()
+    {
+        float gain = _trkGain.Value / 100f;
+        bool muted = _chkMute.Checked;
+        bool nr = _chkNr.Checked;
+        // Apply to all of this user's streams
+        var streams = _client.ListUserStreams(_userId);
+        foreach (var s in streams)
+            _client.SetRemoteStream(_userId, s.StreamId, gain, muted, nr);
+    }
+}