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/Studio/LayoutSource.cs

@@ -0,0 +1,109 @@
+using AcDream.App.UI;
+using AcDream.App.UI.Layout;
+using DatReaderWriter;
+
+namespace AcDream.App.Studio;
+
+/// <summary>Which kind of source the studio is currently previewing.</summary>
+public enum LayoutSourceKind { DatLayout, Markup }
+
+/// <summary>
+/// Wraps the two ways the UI Studio can load a panel to preview:
+/// a LayoutDesc dat id, or a KSML markup file path (Task 6 — unsupported now).
+///
+/// <para>Call <see cref="Load"/> with the current <see cref="StudioOptions"/> to
+/// import the layout and get the root <see cref="UiElement"/>. The result is also
+/// cached in <see cref="CurrentLayout"/> so <see cref="Reload"/> can re-run the same
+/// source without re-reading the options.</para>
+/// </summary>
+public sealed class LayoutSource
+{
+    private readonly DatCollection _dats;
+    private readonly Func<uint, (uint, int, int)> _resolve;
+    private readonly UiDatFont? _datFont;
+
+    public LayoutSourceKind Kind         { get; private set; }
+    public uint?            LayoutId     { get; private set; }
+    public string?          MarkupPath   { get; private set; }
+    public string?          LastError    { get; private set; }
+    public ImportedLayout?  CurrentLayout { get; private set; }
+
+    public LayoutSource(
+        DatCollection dats,
+        Func<uint, (uint, int, int)> resolve,
+        UiDatFont? datFont)
+    {
+        _dats    = dats    ?? throw new ArgumentNullException(nameof(dats));
+        _resolve = resolve ?? throw new ArgumentNullException(nameof(resolve));
+        _datFont = datFont;
+    }
+
+    /// <summary>
+    /// Load the layout described by <paramref name="opts"/>. For a dat layout
+    /// (<see cref="StudioOptions.LayoutId"/> is non-null) calls
+    /// <see cref="LayoutImporter.Import"/>. For a markup path sets
+    /// <see cref="LastError"/> and returns null (Task 6, not yet implemented).
+    ///
+    /// Returns the root <see cref="UiElement"/> on success, or null on failure
+    /// (check <see cref="LastError"/>).
+    /// </summary>
+    public UiElement? Load(StudioOptions opts)
+    {
+        LastError    = null;
+        CurrentLayout = null;
+
+        if (opts.MarkupPath is not null)
+        {
+            Kind       = LayoutSourceKind.Markup;
+            MarkupPath = opts.MarkupPath;
+            LastError  = "markup unsupported (Task 6)";
+            return null;
+        }
+
+        if (opts.LayoutId is null)
+        {
+            LastError = "ui-studio: no layout id or markup path specified.";
+            return null;
+        }
+
+        Kind     = LayoutSourceKind.DatLayout;
+        LayoutId = opts.LayoutId;
+        return LoadDat(opts.LayoutId.Value);
+    }
+
+    /// <summary>Re-run the most-recently-configured source without re-reading options.</summary>
+    public UiElement? Reload()
+    {
+        LastError     = null;
+        CurrentLayout = null;
+
+        if (Kind == LayoutSourceKind.Markup)
+        {
+            LastError = "markup unsupported (Task 6)";
+            return null;
+        }
+
+        if (LayoutId is null)
+        {
+            LastError = "ui-studio: no layout id to reload.";
+            return null;
+        }
+
+        return LoadDat(LayoutId.Value);
+    }
+
+    // ── Private ──────────────────────────────────────────────────────────────────
+
+    private UiElement? LoadDat(uint layoutId)
+    {
+        var imported = LayoutImporter.Import(_dats, layoutId, _resolve, _datFont);
+        if (imported is null)
+        {
+            LastError = $"ui-studio: LayoutDesc 0x{layoutId:X8} not found in dats.";
+            return null;
+        }
+
+        CurrentLayout = imported;
+        return imported.Root;
+    }
+}