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.App/Rendering/GameWindow.cs

@@ -901,6 +901,13 @@ public sealed class GameWindow : IDisposable
                 // the same OnLoad path (see _inputDispatcher field).
                 if (_inputDispatcher is not null)
                 {
+                    // L.0 — settings.json (display + future audio / gameplay /
+                    // chat / character tabs). Coexists with keybinds.json,
+                    // which keeps its own load/save path.
+                    var settingsStore = new AcDream.UI.Abstractions.Panels.Settings.SettingsStore(
+                        AcDream.UI.Abstractions.Panels.Settings.SettingsStore.DefaultPath());
+                    var persistedDisplay = settingsStore.LoadDisplay();
+
                     _settingsVm = new AcDream.UI.Abstractions.Panels.Settings.SettingsVM(
                         persisted: _keyBindings,
                         dispatcher: _inputDispatcher,
@@ -919,6 +926,21 @@ public sealed class GameWindow : IDisposable
                             {
                                 Console.WriteLine($"keybinds: save failed: {ex.Message}");
                             }
+                        },
+                        persistedDisplay: persistedDisplay,
+                        onSaveDisplay: display =>
+                        {
+                            try
+                            {
+                                settingsStore.SaveDisplay(display);
+                                Console.WriteLine(
+                                    "settings: display saved to "
+                                    + AcDream.UI.Abstractions.Panels.Settings.SettingsStore.DefaultPath());
+                            }
+                            catch (Exception ex)
+                            {
+                                Console.WriteLine($"settings: display save failed: {ex.Message}");
+                            }
                         });
                     _settingsPanel = new AcDream.UI.Abstractions.Panels.Settings.SettingsPanel(_settingsVm);
                     _panelHost.Register(_settingsPanel);