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>

tests/AcDream.UI.Abstractions.Tests/Panels/Settings/SettingsStoreTests.cs

@@ -0,0 +1,119 @@
+using System.IO;
+using AcDream.UI.Abstractions.Panels.Settings;
+
+namespace AcDream.UI.Abstractions.Tests.Panels.Settings;
+
+/// <summary>
+/// L.0: <see cref="SettingsStore"/> reads / writes <c>settings.json</c>.
+/// Tests use a temp-file path so they don't touch the user's
+/// %LOCALAPPDATA% file.
+/// </summary>
+public sealed class SettingsStoreTests : System.IDisposable
+{
+    private readonly string _tempPath;
+
+    public SettingsStoreTests()
+    {
+        // Unique per-test file under the system temp dir so parallel test
+        // runners don't clobber each other.
+        _tempPath = Path.Combine(
+            Path.GetTempPath(),
+            $"acdream-settings-test-{System.Guid.NewGuid():N}.json");
+    }
+
+    public void Dispose()
+    {
+        if (File.Exists(_tempPath)) File.Delete(_tempPath);
+    }
+
+    [Fact]
+    public void LoadDisplay_returns_defaults_when_file_is_missing()
+    {
+        var store = new SettingsStore(_tempPath);
+        var loaded = store.LoadDisplay();
+        Assert.Equal(DisplaySettings.Default, loaded);
+    }
+
+    [Fact]
+    public void SaveDisplay_then_LoadDisplay_round_trips_all_fields()
+    {
+        var store = new SettingsStore(_tempPath);
+        var original = new DisplaySettings(
+            Resolution: "2560x1440",
+            Fullscreen: true,
+            VSync: false,
+            FieldOfView: 100f,
+            Gamma: 1.4f,
+            ShowFps: true);
+
+        store.SaveDisplay(original);
+        var loaded = store.LoadDisplay();
+
+        Assert.Equal(original, loaded);
+    }
+
+    [Fact]
+    public void LoadDisplay_falls_back_to_defaults_when_file_is_corrupt()
+    {
+        File.WriteAllText(_tempPath, "{ this is not valid json");
+        var store = new SettingsStore(_tempPath);
+
+        var loaded = store.LoadDisplay();
+
+        Assert.Equal(DisplaySettings.Default, loaded);
+    }
+
+    [Fact]
+    public void LoadDisplay_falls_back_per_field_when_keys_missing()
+    {
+        // Partial file — only resolution set; everything else should
+        // pick up DisplaySettings.Default values.
+        File.WriteAllText(_tempPath, """
+            {
+              "version": 1,
+              "display": { "resolution": "1366x768" }
+            }
+            """);
+        var store = new SettingsStore(_tempPath);
+
+        var loaded = store.LoadDisplay();
+
+        Assert.Equal("1366x768", loaded.Resolution);
+        Assert.Equal(DisplaySettings.Default.Fullscreen, loaded.Fullscreen);
+        Assert.Equal(DisplaySettings.Default.VSync, loaded.VSync);
+        Assert.Equal(DisplaySettings.Default.FieldOfView, loaded.FieldOfView);
+    }
+
+    [Fact]
+    public void SaveDisplay_preserves_unknown_top_level_keys()
+    {
+        // Forward-compat: a newer client may have written sections we
+        // don't know about (audio, gameplay). Saving display must not
+        // delete those, otherwise running an older client would silently
+        // drop the user's other-tab preferences.
+        File.WriteAllText(_tempPath, """
+            {
+              "version": 1,
+              "display": { "resolution": "1280x720" },
+              "audio": { "master": 0.5, "music": 0.7 }
+            }
+            """);
+        var store = new SettingsStore(_tempPath);
+
+        store.SaveDisplay(DisplaySettings.Default with { Resolution = "1920x1080" });
+
+        var raw = File.ReadAllText(_tempPath);
+        Assert.Contains("\"audio\"", raw);
+        Assert.Contains("\"master\"", raw);
+        Assert.Contains("0.5", raw);
+        // And the new display value did get written.
+        Assert.Contains("1920x1080", raw);
+    }
+
+    [Fact]
+    public void DefaultPath_is_under_LocalAppData_acdream()
+    {
+        var path = SettingsStore.DefaultPath();
+        Assert.EndsWith("acdream" + Path.DirectorySeparatorChar + "settings.json", path);
+    }
+}