feat(teleport B): PhysicsEngine.IsLandblockTerrainResident worldReady query

Adds a high-16-bit-prefix landblock residency check used as the teleport
worldReady gate — true once the destination landblock's terrain+cells
have been registered via AddLandblock, regardless of whether the caller
passes a canonical (0xFFFF), cell-resolved, or bare landblock id.

Two TDD tests confirm: false before registration, true after, and
that a cell-resolved id on the same landblock returns true.

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

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

@@ -0,0 +1,34 @@
+using System;
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class PhysicsEngineResidencyTests
+{
+    [Fact]
+    public void IsLandblockTerrainResident_trueOnlyAfterAddLandblock()
+    {
+        var eng = new PhysicsEngine();
+        uint canonical = 0xA9B4FFFFu;          // canonical (streaming) form
+        uint lbId      = canonical & 0xFFFF0000u; // AddLandblock id
+
+        Assert.False(eng.IsLandblockTerrainResident(canonical));
+
+        eng.AddLandblock(lbId, terrain: null!, cells: Array.Empty<CellSurface>(),
+            portals: Array.Empty<PortalPlane>(), worldOffsetX: 0f, worldOffsetY: 0f);
+
+        Assert.True(eng.IsLandblockTerrainResident(canonical));
+    }
+
+    [Fact]
+    public void IsLandblockTerrainResident_matchesOnHigh16_acceptsCellResolvedId()
+    {
+        var eng = new PhysicsEngine();
+        eng.AddLandblock(0xA9B40000u, null!, Array.Empty<CellSurface>(),
+            Array.Empty<PortalPlane>(), 0f, 0f);
+        Assert.True(eng.IsLandblockTerrainResident(0xA9B40019u));  // cell-resolved id, same LB
+        Assert.False(eng.IsLandblockTerrainResident(0xC6A90019u)); // different LB
+    }
+}