M3: reflection-based Inspector — edit any component's fields live

InspectorPanel shows EditorState.Selected's Transform (Position/Scale as
draggable Vector3 widgets, Rotation shown read-only as a quaternion — a
real rotate control needs Euler conversion or a gizmo, out of scope for
just the Inspector) and, below it, every attached Component's public
fields via reflection. No per-component-type drawer code exists anywhere:
int/float/bool/string/Vector3 fields get a live-editable widget, anything
else falls back to a read-only ToString(), and a brand new component type
in any plugin gets an Inspector for free the moment it's attached — that
genericity is the entire reason to do this by reflection instead of a
registry.

Field edits write straight back through FieldInfo.SetValue, same gap
GameObject.AddComponent's doc comment already names: components are plain
fields with nothing to intercept a direct mutation, so this needs no
SystemAccessScope declaration any more than a hand-written `component.
Value = 5` would.

EditorPlugin now defaults EditorState.Selected to the first root once,
if nothing's been clicked yet — an empty Inspector on every fresh launch
had nothing useful to show. Verified by screenshot: Inspector displays
"Quad"'s real Position (0,0,0) and Scale (2,2,1), matching scene.json
exactly, plus a QuadRenderer header correctly showing "(no fields)" since
it has none yet.

Full suite still green: 65 tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N1qPfzq8TDCUMFMV3UwV5N

plugins/engine.editor/Engine.Editor/EditorPlugin.cs

@@ -60,12 +60,21 @@ public sealed class EditorPlugin : IPlugin
     {
         _controller!.Update(_time!.DeltaTime);
 
+        // Nothing selected yet and there's something to select: default to
+        // the first root rather than opening on an empty, useless
+        // Inspector. Only fires once — any real click overwrites it, and
+        // it never fights a deliberate deselect because there's no way to
+        // deselect yet.
+        if (_state.Selected is null && world.Roots.Count > 0)
+            _state.Selected = world.Roots[0];
+
         ImGui.Begin("Lingua Editor");
         ImGui.Text($"FPS: {1f / MathF.Max(_time.DeltaTime, 0.0001f):F0}");
         ImGui.Text($"Frame: {_time.FrameCount}");
         ImGui.End();
 
         HierarchyPanel.Draw(world, _state);
+        InspectorPanel.Draw(_state);
 
         _controller.Render();
     }