feat(studio): StudioWindow + LayoutSource — render a dat panel standalone

Task 2 of the acdream UI Studio plan. Adds three new files under
src/AcDream.App/Studio/ and one new test file:

- StudioOptions.cs: record + Parse() for the ui-studio CLI args (positional
  dat dir or ACDREAM_DAT_DIR, --layout 0xNNNN, --markup <path>; defaults to
  vitals 0x2100006C when neither layout nor markup given).
- LayoutSource.cs: wraps LayoutImporter.Import for dat-backed layouts; markup
  path sets LastError "markup unsupported (Task 6)" and returns null for now.
  Exposes Kind / LayoutId / MarkupPath / LastError / CurrentLayout / Reload().
- StudioWindow.cs: Silk.NET 1280×720 GL 4.3 window; boots RenderBootstrap,
  wires input to UiHost, loads the panel via LayoutSource, adds the root to
  UiHost.Root.AddChild. QualitySettings resolved the same way GameWindow.Run()
  does (SettingsStore → QualitySettings.From → WithEnvOverrides).
- Program.cs: ui-studio dispatch at the top of top-level statements (before
  the dat-dir parse) so `dotnet run -- ui-studio` routes to StudioWindow.
- LayoutSourceTests.cs: dat-gated test (skips when dats absent); verifies that
  the vitals LayoutDesc (0x2100006C) loads, root is non-null, byId contains the
  vitals root element (0x100005F9), and Kind == DatLayout. Passes (1/1 with dats
  present; silently skips on CI).

Note: the spec asserts FindElement(0x2100006Cu) — that id is the LayoutDesc
dat id, not a widget element id. The actual vitals root element id is 0x100005F9
(confirmed from the vitals_2100006C.json fixture). The test uses the correct
element id and documents the discrepancy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Program.cs

@@ -4,6 +4,14 @@ using AcDream.App.Rendering;
 using AcDream.Core.Plugins;
 using Serilog;
 
+if (args.Length >= 1 && args[0] == "ui-studio")
+{
+    var so = AcDream.App.Studio.StudioOptions.Parse(args[1..]);
+    using var sw = new AcDream.App.Studio.StudioWindow(so);
+    sw.Run();
+    return 0;
+}
+
 Log.Logger = new LoggerConfiguration()
     .MinimumLevel.Debug()
     .WriteTo.Console()