feat(ui): Display tab + settings.json persistence — first non-keybind tab lands

Phase L.0 (cont.) — first concrete tab on the new Settings shell, in
the Easy-wins build order agreed in the brainstorm
(Display → Audio → Gameplay → Chat → Character).

DisplaySettings (immutable record): Resolution / Fullscreen / VSync /
FieldOfView (30-120°) / Gamma (0.5-2.0) / ShowFps. Six common 16:9
resolutions in the dropdown. Defaults: 1920×1080, windowed, vsync on,
75° FOV, gamma 1.0, FPS off — matches the brainstorm UX agreement.

SettingsStore: JSON persistence at %LOCALAPPDATA%\acdream\settings.json
(coexists with keybinds.json — own load/save path stays put, no
migration needed). LoadDisplay falls back per-field when keys are
missing (partial-file tolerant) and falls back to defaults when the
file is corrupt or the JSON is unparseable. SaveDisplay round-trips
preserved — unknown top-level keys (e.g. an `audio` section written
by a future client) are kept on save so older builds don't silently
drop newer-tab data.

SettingsVM gains a parallel display-state machine: persistedDisplay +
draftDisplay, SetDisplay mutator, HasUnsavedChanges checks both
keybinds and display deltas, Save/Cancel/ResetAll cover both
atomically from the user's POV (one Save commits everything, one
Cancel reverts everything). Constructor signature extends with two
new params; existing keybinds-only callers updated.

SettingsPanel.RenderDisplayTab replaces the L.0-shell placeholder —
Combo for resolution, Checkboxes for fullscreen/vsync/show-fps,
SliderFloat for FOV + gamma. Live-preview note in the panel body
matches the agreed UX: FOV + gamma update visibly while the user
drags; resolution / fullscreen / vsync apply on Save (live preview
would be too jarring).

GameWindow wires SettingsStore into the existing SettingsVM construct
site — load on startup, save on each tab Save. Errors print to
console and don't crash the panel.

19 new tests:
 · DisplaySettings record (4) — defaults pinned, value equality, with-
   expressions, AvailableResolutions sorted ascending
 · SettingsStore (6) — round trip, missing-file → defaults, corrupt-
   file → defaults, partial-file → per-field fallback, unknown-key
   preservation, DefaultPath shape
 · SettingsVM display (6) — initial draft tracks persisted, SetDisplay
   marks dirty, Save invokes display callback, Cancel reverts,
   ResetAllToDefaults covers display, Save-then-Cancel is no-op
 · SettingsPanel display tab (3) — widgets render only when active,
   resolution combo uses AvailableResolutions, no Combo emitted on
   inactive tabs

dotnet build green (0 warnings); dotnet test 1,246 / 1,246 green
(243 Core.Net + 330 UI.Abstractions + 673 Core).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.UI.Abstractions/Panels/Settings/SettingsVM.cs

@@ -30,6 +30,12 @@ public sealed class SettingsVM
     private readonly InputDispatcher _dispatcher;
     private readonly Action<KeyBindings> _onSave;
 
+    // L.0 — Display tab. Treated as a single immutable record; mutation
+    // through SetDisplay clones via with-expressions on the panel side.
+    private DisplaySettings _displayPersisted;
+    private DisplaySettings _displayDraft;
+    private readonly Action<DisplaySettings> _onSaveDisplay;
+
     /// <summary>The action currently being rebound, or null when idle.</summary>
     public InputAction? RebindInProgress { get; private set; }
 
@@ -50,14 +56,38 @@ public sealed class SettingsVM
     /// <summary>True iff the draft differs structurally from the
     /// persisted snapshot. Used to grey out the Save button when no
     /// rebinds are pending.</summary>
-    public bool HasUnsavedChanges => !KeyBindingsEqual(_persisted, _draft);
+    public bool HasUnsavedChanges
+        => !KeyBindingsEqual(_persisted, _draft)
+           || _displayPersisted != _displayDraft;
 
-    public SettingsVM(KeyBindings persisted, InputDispatcher dispatcher, Action<KeyBindings> onSave)
+    /// <summary>The current Display draft. Panel reads from here;
+    /// mutation goes through <see cref="SetDisplay"/>.</summary>
+    public DisplaySettings DisplayDraft => _displayDraft;
+
+    public SettingsVM(
+        KeyBindings persisted,
+        InputDispatcher dispatcher,
+        Action<KeyBindings> onSave,
+        DisplaySettings persistedDisplay,
+        Action<DisplaySettings> onSaveDisplay)
     {
-        _persisted  = persisted ?? throw new ArgumentNullException(nameof(persisted));
-        _dispatcher = dispatcher ?? throw new ArgumentNullException(nameof(dispatcher));
-        _onSave     = onSave ?? throw new ArgumentNullException(nameof(onSave));
-        _draft      = CloneBindings(persisted);
+        _persisted        = persisted        ?? throw new ArgumentNullException(nameof(persisted));
+        _dispatcher       = dispatcher       ?? throw new ArgumentNullException(nameof(dispatcher));
+        _onSave           = onSave           ?? throw new ArgumentNullException(nameof(onSave));
+        _displayPersisted = persistedDisplay ?? throw new ArgumentNullException(nameof(persistedDisplay));
+        _onSaveDisplay    = onSaveDisplay    ?? throw new ArgumentNullException(nameof(onSaveDisplay));
+        _draft            = CloneBindings(persisted);
+        _displayDraft     = persistedDisplay;
+    }
+
+    /// <summary>
+    /// Replace the entire Display draft with <paramref name="value"/>.
+    /// Panel calls this with a <c>DisplayDraft with { Field = newValue }</c>
+    /// so each widget edits exactly one field at a time.
+    /// </summary>
+    public void SetDisplay(DisplaySettings value)
+    {
+        _displayDraft = value ?? throw new ArgumentNullException(nameof(value));
     }
 
     /// <summary>
@@ -160,32 +190,42 @@ public sealed class SettingsVM
     }
 
     /// <summary>
-    /// Replace the entire draft with <see cref="KeyBindings.RetailDefaults"/>.
+    /// Replace the keybinds draft with <see cref="KeyBindings.RetailDefaults"/>
+    /// AND the display draft with <see cref="DisplaySettings.Default"/>.
+    /// "Reset all" applies to every tab — it's the user's escape hatch
+    /// when they've gotten lost.
     /// </summary>
     public void ResetAllToDefaults()
     {
-        _draft = KeyBindings.RetailDefaults();
+        _draft        = KeyBindings.RetailDefaults();
+        _displayDraft = DisplaySettings.Default;
     }
 
     /// <summary>
-    /// Commit the draft via the onSave callback supplied at
-    /// construction. After save the draft becomes the new persisted
-    /// snapshot — <see cref="HasUnsavedChanges"/> resets to false.
+    /// Commit both keybinds + display drafts via the onSave callbacks
+    /// supplied at construction. After save the drafts become the new
+    /// persisted snapshots — <see cref="HasUnsavedChanges"/> resets to
+    /// false. Each callback is invoked exactly once per Save; if the
+    /// caller wants atomicity across both files it has to handle it
+    /// outside the VM.
     /// </summary>
     public void Save()
     {
         _onSave(_draft);
-        _persisted = CloneBindings(_draft);
+        _onSaveDisplay(_displayDraft);
+        _persisted        = CloneBindings(_draft);
+        _displayPersisted = _displayDraft;
     }
 
     /// <summary>
-    /// Revert the draft to the persisted snapshot and clear any
+    /// Revert all drafts to their persisted snapshots and clear any
     /// in-flight rebind state. Used by the panel's "Cancel" button and
     /// when the user closes the settings window without saving.
     /// </summary>
     public void Cancel()
     {
-        _draft = CloneBindings(_persisted);
+        _draft        = CloneBindings(_persisted);
+        _displayDraft = _displayPersisted;
         CancelRebind();
     }