feat(ui): Gameplay tab — 14 retail CharacterOption-derived toggles

Phase L.0 (cont.) — third tab on the Settings shell, in the Easy-wins
build order. Subset of retail's CharacterOption + CharacterOptions2
bitfield flags ported as bools (see acclient.h:3404+ enum). Local-
only this phase per the brainstorm — server sync deferred to a
later phase that will marshal the draft into the retail
CharacterOption packet.

GameplaySettings record exposes 14 named flags grouped by usage:

 · Combat: AutoTarget, AutoRepeatAttack, ToggleRun, AdvancedCombatUI,
   VividTargetingIndicator
 · Display: ShowTooltips, SideBySideVitals, CoordinatesOnRadar,
   SpellDuration, ShowHelm, ShowCloak
 · Interface: AllowGive, LockUI, UseMouseTurning

Retail names + bit values are documented in field-level comments so
the future server-sync phase has a 1:1 mapping. Defaults are
typical-user starting points (NOT bit-exact to retail's
0x50C4A54A / 0x948700 masks); class-level remarks call out that
defaults will be re-anchored to retail values once the wire-format
is the load-bearing source.

SettingsStore grows LoadGameplay / SaveGameplay using the existing
SaveSection generic helper (added in the audio commit). All three
non-keybind sections (display, audio, gameplay) now coexist in
settings.json with non-destructive cross-section saves — verified
by a new "all three sections coexist" round-trip test.

SettingsVM grows the parallel gameplay state machine
(gameplayPersisted / gameplayDraft / SetGameplay / onSaveGameplay).
HasUnsavedChanges, Save, Cancel, ResetAllToDefaults all cover
gameplay too. Constructor signature adds two more params; existing
call sites (App startup + tests) updated.

SettingsPanel.RenderGameplayTab replaces the L.0-shell placeholder —
14 Checkbox calls grouped under three Text+Separator headers, plus
a footer note explaining the local-only-this-phase scope. The
"Coming soon" placeholder test was retargeted from "Gameplay" to
"Chat" since Gameplay is no longer a placeholder.

GameWindow construction site loads gameplay on startup + writes via
the SettingsStore on Save. Server-sync packet wiring is left as a
TODO comment in the onSaveGameplay callback (next phase, after the
protocol round-trip is in place).

14 new tests:
 · GameplaySettings record (3) — defaults pinned, value equality,
   with-expressions
 · SettingsStore gameplay (4) — missing-file → defaults, round-trip,
   partial-file fallback, all-three-sections coexist
 · SettingsVM gameplay (5) — initial draft, SetGameplay marks dirty,
   Save invokes callback, Cancel reverts, ResetAllToDefaults covers
 · SettingsPanel gameplay tab (2) — 8 spot-checked Checkboxes render
   only when active

dotnet build green (0 warnings); dotnet test 1,276 / 1,276 green
(243 Core.Net + 360 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

@@ -180,4 +180,64 @@ public sealed class SettingsStoreTests : System.IDisposable
         Assert.Equal(0.1f, store.LoadAudio().Music);
         Assert.True(store.LoadDisplay().ShowFps);
     }
+
+    // -- Gameplay section round-trip --------------------------------------
+
+    [Fact]
+    public void LoadGameplay_returns_defaults_when_file_is_missing()
+    {
+        var store = new SettingsStore(_tempPath);
+        Assert.Equal(GameplaySettings.Default, store.LoadGameplay());
+    }
+
+    [Fact]
+    public void SaveGameplay_then_LoadGameplay_round_trips_all_fields()
+    {
+        var store = new SettingsStore(_tempPath);
+        var original = GameplaySettings.Default with
+        {
+            AutoTarget = false,
+            AdvancedCombatUI = true,
+            ShowHelm = false,
+            LockUI = true,
+            UseMouseTurning = true,
+        };
+
+        store.SaveGameplay(original);
+        var loaded = store.LoadGameplay();
+
+        Assert.Equal(original, loaded);
+    }
+
+    [Fact]
+    public void LoadGameplay_falls_back_per_field_when_keys_missing()
+    {
+        File.WriteAllText(_tempPath, """
+            {
+              "version": 1,
+              "gameplay": { "lockUI": true }
+            }
+            """);
+        var store = new SettingsStore(_tempPath);
+
+        var loaded = store.LoadGameplay();
+
+        Assert.True(loaded.LockUI);
+        Assert.Equal(GameplaySettings.Default.AutoTarget, loaded.AutoTarget);
+        Assert.Equal(GameplaySettings.Default.ShowHelm, loaded.ShowHelm);
+    }
+
+    [Fact]
+    public void All_three_sections_coexist_in_one_settings_json()
+    {
+        var store = new SettingsStore(_tempPath);
+        store.SaveDisplay(DisplaySettings.Default with { Resolution = "2560x1440" });
+        store.SaveAudio(AudioSettings.Default with { Master = 0.5f });
+        store.SaveGameplay(GameplaySettings.Default with { LockUI = true });
+
+        // All three load correctly from the same file.
+        Assert.Equal("2560x1440", store.LoadDisplay().Resolution);
+        Assert.Equal(0.5f, store.LoadAudio().Master);
+        Assert.True(store.LoadGameplay().LockUI);
+    }
 }