feat(studio): dump LayoutSource — preview any retail window from the UI dump

Adds Task 4b: a second load path for the UI Studio that reads the committed
retail UI layout dump (docs/research/2026-06-25-retail-ui-layout-dump.json)
and renders any of the 26 retail windows as a static sprite hierarchy.

New files:
- src/AcDream.App/Studio/UiDumpModel.cs — POCOs + System.Text.Json parse of
  the dump (UiDump, DumpPanel, DumpNode, DumpRect, DumpStateSet, DumpImage,
  DumpState + UiDumpModel static helpers: Parse, ListSlugs, PickImageId).
- src/AcDream.App/Studio/DumpLayout.cs — DumpLayout.Load(path, slug, resolve,
  out err): parses the dump, finds the panel by slug, builds a UiElement tree.
  Internal DumpSpriteElement draws its sprite via DrawSprite (not reusing
  UiDatElement — avoids the ElementInfo/StateMedia dat-import dependency for
  this static mockup). DumpGroupElement is a transparent container for Group
  nodes. Rect basis is ABSOLUTE in the dump (verified: inventory root at
  absolute x=500 and its children also start at x≈500 — child offset from
  parent is 0–50px, not 500px); DumpLayout subtracts parent rect to produce
  parent-relative Left/Top for each child.
- tests/AcDream.App.Tests/Studio/DumpLayoutTests.cs — 5 tests covering:
  inventory load with 0x100001D5 check + >= 40 nodes, unknown slug → null+err,
  root at origin, children are parent-relative, all 26 slugs smoke-load.

Modified files:
- StudioOptions: adds DumpSlug + DumpFile fields; --dump <slug> and
  --dump-file <path> args; ResolveDumpFile() walks up to the solution root
  to find the default dump JSON (mirrors ConformanceDats.SolutionRoot()).
  --dump suppresses the default vitals layout so the two modes are exclusive.
- StudioWindow.OnLoad: when DumpSlug is set, loads via DumpLayout (no
  FixtureProvider, no controllers — static structure only); else falls
  through to the existing LayoutSource + FixtureProvider path.

Results: DumpLayoutTests 5/5 passed; full AcDream.App.Tests 609 passed, 2
skipped (same as before); dotnet build green, 0 warnings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AcDream.App/Studio/StudioWindow.cs

@@ -117,8 +117,34 @@ public sealed class StudioWindow : IDisposable
             _stack.UiHost.WireKeyboard(kb);
 
         // Load the panel described by options and add it to the UI tree.
+        // Task 4b: --dump <slug> uses DumpLayout (static retail mockup, no controllers).
+        // All other modes use LayoutSource + FixtureProvider (production path).
         _source = new LayoutSource(_dats, _stack.ResolveChrome, _stack.VitalsDatFont);
-        var root = _source.Load(_opts);
+        UiElement? root;
+        if (_opts.DumpSlug is not null)
+        {
+            var dumpFile = _opts.ResolveDumpFile();
+            if (dumpFile is null)
+            {
+                Console.Error.WriteLine("[studio] --dump: retail UI dump file not found. " +
+                    "Expected docs/research/2026-06-25-retail-ui-layout-dump.json in the source tree, " +
+                    "or pass --dump-file <path>.");
+                root = null;
+            }
+            else
+            {
+                root = DumpLayout.Load(dumpFile, _opts.DumpSlug, _stack.ResolveChrome, out var dumpErr);
+                if (root is null)
+                    Console.Error.WriteLine($"[studio] dump load failed: {dumpErr}");
+            }
+        }
+        else
+        {
+            root = _source.Load(_opts);
+            if (root is null)
+                Console.Error.WriteLine($"[studio] panel load failed: {_source.LastError}");
+        }
+
         _panelRoot = root;
         if (root is not null)
         {
@@ -126,15 +152,14 @@ public sealed class StudioWindow : IDisposable
 
             // Task 4: populate the panel with sample data via production controllers,
             // so inventory / vitals / toolbar panels render with plausible content.
-            if (_source.CurrentLayout is not null)
+            // Dump source is static — no FixtureProvider needed.
+            if (_opts.DumpSlug is null && _source.CurrentLayout is not null)
             {
                 uint layoutId = _opts.LayoutId ?? 0x2100006Cu;
                 _objects = SampleData.BuildObjectTable();
                 FixtureProvider.Populate(layoutId, _source.CurrentLayout, _stack, _objects, _dats);
             }
         }
-        else
-            Console.Error.WriteLine($"[studio] panel load failed: {_source.LastError}");
 
         // Task 3: ImGui IDE. The studio is always "devtools" — no gate flag needed.
         _imgui     = new AcDream.UI.ImGui.ImGuiBootstrapper(gl, _window!, input);