feat(physics #145): Slice 2a — PhysicsBody carries CellPosition; setter delta-syncs into the cell frame

Add CellPosition (retail Position type) alongside the world Vector3 Position.
The Position setter mirrors each world delta into the cell-local origin and
calls AdjustToOutside only when the local coord crosses a landblock boundary
([0,192) on X or Y), so the within-block cell id is preserved from the wire
seed. SnapToCell seeds both positions from the wire's (cell, local) pair
verbatim — no streaming center involved. Unseeded bodies (ObjCellId==0) and
indoor cells are no-ops in the delta path. UpdatePhysicsInternal's existing
`Position +=` desugars through the new setter automatically; no call sites
changed. 4 new unit tests; full Core suite 1526 passed / 0 failed / 2 skipped.

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

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

@@ -0,0 +1,51 @@
+using System.Numerics;
+using AcDream.Core.Physics;
+using Xunit;
+
+namespace AcDream.Core.Tests.Physics;
+
+public class PhysicsBodyCellSyncTests
+{
+    [Fact]
+    public void SnapToCell_SeedsCellPosition_AndWorldPosition()
+    {
+        var body = new PhysicsBody();
+        body.SnapToCell(0xC95B0001u, worldPos: new Vector3(1000f, 2000f, 12f), cellLocal: new Vector3(14.8f, 0.3f, 12f));
+        Assert.Equal(0xC95B0001u, body.CellPosition.ObjCellId);
+        Assert.Equal(new Vector3(14.8f, 0.3f, 12f), body.CellPosition.Frame.Origin);
+        Assert.Equal(new Vector3(1000f, 2000f, 12f), body.Position);
+    }
+
+    [Fact]
+    public void PositionDelta_MirrorsIntoCellPosition_WithinLandblock()
+    {
+        var body = new PhysicsBody();
+        body.SnapToCell(0xC95B0001u, new Vector3(1000f, 2000f, 12f), new Vector3(100f, 50f, 12f));
+        body.Position += new Vector3(5f, 3f, 0f);   // move within the landblock
+        Assert.Equal(new Vector3(105f, 53f, 12f), body.CellPosition.Frame.Origin);
+        Assert.Equal(0xC95B0001u, body.CellPosition.ObjCellId);   // no [0,192) crossing → same cell
+        Assert.Equal(new Vector3(1005f, 2003f, 12f), body.Position); // world still authoritative + moved
+    }
+
+    [Fact]
+    public void PositionDelta_AcrossSouthLandblockEdge_BumpsCellAndRewraps()
+    {
+        var body = new PhysicsBody();
+        // local Y just above 0 in 0xC95B (lbY=0x5B); move south past 0 → south neighbour 0xC95A, local Y near 192.
+        body.SnapToCell(0xC95B0001u, new Vector3(1000f, 2000f, 12f), new Vector3(100f, 0.3f, 12f));
+        body.Position += new Vector3(0f, -1f, 0f);
+        int lbY = (int)((body.CellPosition.ObjCellId >> 16) & 0xFFu);
+        Assert.Equal(0x5A, lbY);                                     // bumped to south neighbour
+        Assert.InRange(body.CellPosition.Frame.Origin.Y, 190f, 192f); // re-wrapped near top of southern block
+    }
+
+    [Fact]
+    public void PositionDelta_WithoutSeed_LeavesCellPositionDefault()
+    {
+        // A body that was never SnapToCell'd (e.g. a remote entity) must NOT get
+        // a synthesized cell — CellPosition stays default, no AdjustToOutside.
+        var body = new PhysicsBody();
+        body.Position += new Vector3(10f, 10f, 0f);
+        Assert.Equal(0u, body.CellPosition.ObjCellId);
+    }
+}