feat(slice2): add PhysicsEngine.IsLandblockLoaded — readiness gate §3.4

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

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -31,6 +31,15 @@ public sealed class PhysicsEngine
     /// <summary>Number of registered landblocks (diagnostic).</summary>
     public int LandblockCount => _landblocks.Count;
 
+    /// <summary>
+    /// Returns true iff a landblock with this id has been registered via
+    /// <see cref="AddLandblock"/>. Because <see cref="AddLandblock"/> registers
+    /// terrain + collision cells + portals atomically, ContainsKey implies the
+    /// collision mesh is present and resolvable.
+    /// </summary>
+    public bool IsLandblockLoaded(uint landblockId) =>
+        _landblocks.ContainsKey(landblockId);
+
     /// <summary>
     /// Cell-based spatial index for static object collision.
     /// Populated during landblock streaming; queried by the Transition system.

tests/AcDream.Core.Tests/Physics/PhysicsEngineIsLandblockLoadedTests.cs

@@ -0,0 +1,57 @@
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class PhysicsEngineIsLandblockLoadedTests
+{
+    private static TerrainSurface MakeFlatTerrain()
+    {
+        var heights = new byte[81];
+        Array.Fill(heights, (byte)50);
+        var heightTable = new float[256];
+        for (int i = 0; i < 256; i++) heightTable[i] = i * 1f;
+        return new TerrainSurface(heights, heightTable);
+    }
+
+    [Fact]
+    public void IsLandblockLoaded_AbsentId_ReturnsFalse()
+    {
+        var engine = new PhysicsEngine();
+        Assert.False(engine.IsLandblockLoaded(0xA9B4FFFFu));
+    }
+
+    [Fact]
+    public void IsLandblockLoaded_AddedId_ReturnsTrue()
+    {
+        var engine = new PhysicsEngine();
+        engine.AddLandblock(0xA9B4FFFFu, MakeFlatTerrain(),
+            Array.Empty<CellSurface>(), Array.Empty<PortalPlane>(),
+            worldOffsetX: 0f, worldOffsetY: 0f);
+
+        Assert.True(engine.IsLandblockLoaded(0xA9B4FFFFu));
+    }
+
+    [Fact]
+    public void IsLandblockLoaded_AnotherIdAbsent_ReturnsFalse()
+    {
+        var engine = new PhysicsEngine();
+        engine.AddLandblock(0xA9B4FFFFu, MakeFlatTerrain(),
+            Array.Empty<CellSurface>(), Array.Empty<PortalPlane>(),
+            worldOffsetX: 0f, worldOffsetY: 0f);
+
+        Assert.False(engine.IsLandblockLoaded(0xDEADBEEFu));
+    }
+
+    [Fact]
+    public void IsLandblockLoaded_AfterRemove_ReturnsFalse()
+    {
+        var engine = new PhysicsEngine();
+        engine.AddLandblock(0xA9B4FFFFu, MakeFlatTerrain(),
+            Array.Empty<CellSurface>(), Array.Empty<PortalPlane>(),
+            worldOffsetX: 0f, worldOffsetY: 0f);
+        engine.RemoveLandblock(0xA9B4FFFFu);
+
+        Assert.False(engine.IsLandblockLoaded(0xA9B4FFFFu));
+    }
+}