Add MrGameEng.Inspector: in-engine ECS debugger overlay

Introduce a Chrome-DevTools-style ECS inspector in the UI library. The
headless core (ComponentReflector + EcsInspector) enumerates the store's
entities grouped by archetype, reflects a selected entity's components and
fields, and writes simple scalar edits (number/bool/enum) back through a
generic AddComponent — all unit-tested without a GPU. The Myra overlay
(EcsInspectorUi + InspectorSystems) adds a side panel with an
archetypes -> entities -> fields tree, an editable field view, a
world-pick mode with a selection highlight, and a renderer performance
tab; wired via scene.UseInspector(renderer), toggled with F1.

The UI library now references Graphics (for picking, renderer timings and
Transform2D/Sprite). Docs updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/MrGameEng.UI.Tests/EcsInspectorTests.cs

@@ -0,0 +1,144 @@
+using Friflo.Engine.ECS;
+using MrGameEng.Inspector;
+using Xunit;
+
+namespace MrGameEng.Inspector.Tests;
+
+public class EcsInspectorTests
+{
+    private enum Mode
+    {
+        Idle,
+        Run,
+    }
+
+    private struct Probe : IComponent
+    {
+        public int Count;
+        public bool Active;
+        public Mode State;
+    }
+
+    private struct Marker : IComponent
+    {
+        public int Value;
+    }
+
+    [Fact]
+    public void Archetypes_GroupsEntitiesByComponentSet()
+    {
+        var store = new EntityStore();
+        store.CreateEntity(new Probe { Count = 1 });
+        store.CreateEntity(new Probe { Count = 2 });
+        store.CreateEntity(new Marker { Value = 9 });
+        var inspector = new EcsInspector(store);
+
+        var rows = inspector.Archetypes();
+
+        var probeRow = Single(rows, r => r.Signature.Contains("Probe"));
+        Assert.Equal(2, probeRow.Count);
+        Assert.Contains(rows, r => r.Signature.Contains("Marker") && r.Count == 1);
+    }
+
+    [Fact]
+    public void EntitiesOf_ReturnsIdsOfArchetype()
+    {
+        var store = new EntityStore();
+        var a = store.CreateEntity(new Probe { Count = 1 });
+        var b = store.CreateEntity(new Probe { Count = 2 });
+        var inspector = new EcsInspector(store);
+
+        var probeRow = Single(inspector.Archetypes(), r => r.Signature.Contains("Probe"));
+        var ids = inspector.EntitiesOf(probeRow.Index);
+
+        Assert.Contains(a.Id, ids);
+        Assert.Contains(b.Id, ids);
+        Assert.Equal(2, ids.Count);
+    }
+
+    [Fact]
+    public void Inspect_ReturnsComponentFields()
+    {
+        var store = new EntityStore();
+        var entity = store.CreateEntity(
+            new Probe
+            {
+                Count = 5,
+                Active = true,
+                State = Mode.Run,
+            }
+        );
+        var inspector = new EcsInspector(store);
+
+        var view = Single(inspector.Inspect(entity.Id), c => c.TypeName == "Probe");
+
+        Assert.Equal("5", Field(view, "Count").Value);
+        Assert.Equal("true", Field(view, "Active").Value);
+        Assert.Equal("Run", Field(view, "State").Value);
+    }
+
+    [Fact]
+    public void SetField_WritesValueBackToEntity()
+    {
+        var store = new EntityStore();
+        var entity = store.CreateEntity(new Probe { Count = 5 });
+        var inspector = new EcsInspector(store);
+
+        var ok = inspector.SetField(entity.Id, typeof(Probe), "Count", "42");
+
+        Assert.True(ok);
+        Assert.Equal(42, entity.GetComponent<Probe>().Count);
+    }
+
+    [Fact]
+    public void SetField_RejectsInvalidValue()
+    {
+        var store = new EntityStore();
+        var entity = store.CreateEntity(new Probe { Count = 5 });
+        var inspector = new EcsInspector(store);
+
+        Assert.False(inspector.SetField(entity.Id, typeof(Probe), "Count", "bad"));
+        Assert.Equal(5, entity.GetComponent<Probe>().Count);
+    }
+
+    [Fact]
+    public void Search_FiltersArchetypesBySignature()
+    {
+        var store = new EntityStore();
+        store.CreateEntity(new Probe());
+        store.CreateEntity(new Marker());
+        var inspector = new EcsInspector(store);
+
+        inspector.SetSearch("Marker");
+
+        var rows = inspector.Archetypes();
+        Assert.Single(rows);
+        Assert.Contains("Marker", rows[0].Signature);
+    }
+
+    private static T Single<T>(IReadOnlyList<T> items, Func<T, bool> predicate)
+    {
+        foreach (var item in items)
+        {
+            if (predicate(item))
+            {
+                return item;
+            }
+        }
+
+        throw new Xunit.Sdk.XunitException("no matching item");
+    }
+
+    private static FieldRow Field(ComponentView view, string name)
+    {
+        foreach (var row in view.Fields)
+        {
+            if (row.Name == name)
+            {
+                return row;
+            }
+        }
+
+        throw new Xunit.Sdk.XunitException($"field {name} not found");
+    }
+}