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>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -31,6 +31,20 @@ public sealed class PhysicsEngine
     /// <summary>Number of registered landblocks (diagnostic).</summary>
     public int LandblockCount => _landblocks.Count;
 
+    /// <summary>
+    /// True once the landblock covering <paramref name="cellOrLandblockId"/> has had its
+    /// terrain + cells registered via <see cref="AddLandblock"/>. Accepts a canonical
+    /// (0xFFFF) id, a cell-resolved id, or a bare landblock id — compares on the high 16
+    /// bits. This is the teleport "worldReady" gate (the destination is grounded).
+    /// </summary>
+    public bool IsLandblockTerrainResident(uint cellOrLandblockId)
+    {
+        uint prefix = cellOrLandblockId & 0xFFFF0000u;
+        foreach (var key in _landblocks.Keys)
+            if ((key & 0xFFFF0000u) == prefix) return true;
+        return false;
+    }
+
     /// <summary>
     /// Cell-based spatial index for static object collision.
     /// Populated during landblock streaming; queried by the Transition system.

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
+    }
+}