feat(A.5 T22.5): QualityPreset schema + tests (commit 1/2)

Add QualityPreset enum + QualitySettings readonly record struct with
From(preset) table and WithEnvOverrides() env-var override layer.
Four presets (Low/Medium/High/Ultra) drive NearRadius, FarRadius,
MsaaSamples, AnisotropicLevel, AlphaToCoverage, MaxCompletionsPerFrame.
Env vars (ACDREAM_NEAR_RADIUS, ACDREAM_FAR_RADIUS, ACDREAM_MSAA_SAMPLES,
ACDREAM_ANISOTROPIC, ACDREAM_A2C, ACDREAM_MAX_COMPLETIONS_PER_FRAME)
override individual preset fields for dev spot-testing.

DisplaySettings gains a Quality: QualityPreset field (default High);
SettingsStore persists/loads it under display."quality" as an enum
name string with Enum.TryParse fallback. 12 new QualityPresetTests
cover the preset table (radii, msaa, aniso, a2c, completions) and all
six env-var override paths. 415 UI.Abstractions tests passing.

Wiring into GameWindow / WbDrawDispatcher / TerrainAtlas follows in
commit 2 of this task.

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

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

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using AcDream.UI.Abstractions.Settings;
 
 namespace AcDream.UI.Abstractions.Panels.Settings;
 
@@ -20,7 +21,8 @@ public sealed record DisplaySettings(
     bool VSync,
     float FieldOfView,
     float Gamma,
-    bool ShowFps)
+    bool ShowFps,
+    QualityPreset Quality)
 {
     /// <summary>Values used on first launch / when settings.json is absent.
     /// All defaults pinned to the pre-L.0 runtime state — Resolution
@@ -35,7 +37,8 @@ public sealed record DisplaySettings(
         VSync:        false,
         FieldOfView:  60f,
         Gamma:        1.0f,
-        ShowFps:      true);
+        ShowFps:      true,
+        Quality:      QualityPreset.High);
 
     /// <summary>16:9 resolution presets offered in the dropdown.</summary>
     public static IReadOnlyList<string> AvailableResolutions { get; } = new[]