test(physics #145): continuous-tracking test — refutes the review staleness claim

Adversarial review flagged a possible CellPosition staleness on indoor->outdoor
transition. Verified against source: false positive (SnapToCell isn't called on
building entry; the body is world-space so the delta is frame-invariant; the anchor
disengages indoors). Added a test proving CellPosition tracks the outdoor cell under
the world position across a multi-cell, cross-landblock walk and stays canonical.
Core cell-sync 6/6.

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

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

@@ -65,4 +65,38 @@ public class PhysicsBodyCellSyncTests
         body.Position += new Vector3(10f, 10f, 0f);
         Assert.Equal(0u, body.CellPosition.ObjCellId);
     }
+
+    [Fact]
+    public void ContinuousTracking_LongWalkAcrossCellsAndBlock_NeverGoesStale()
+    {
+        // Refutes the "CellPosition goes stale" concern (incl. the indoor-transit case):
+        // because the Position setter re-derives the cell via AdjustToOutside on EVERY
+        // write, CellPosition continuously tracks the outdoor cell under the body's WORLD
+        // position. The body is always world-space, so the world delta is frame-invariant —
+        // there is no separate "interior" frame to desync. On any exit it is the correct
+        // cell, never stale.
+        var body = new PhysicsBody();
+        body.SnapToCell(0xC95B0001u, new Vector3(1000f, 2000f, 12f), new Vector3(10f, 10f, 12f));
+
+        // A winding walk: east within the block, then north across the 192 m boundary.
+        foreach (var step in new[]
+        {
+            new Vector3(50f, 0f, 0f),
+            new Vector3(50f, 0f, 0f),
+            new Vector3(0f, 100f, 0f),
+            new Vector3(0f, 100f, 0f),   // local Y 110 → 210, crosses north into 0xC95C
+        })
+            body.Position += step;
+
+        // local accum (10,10)+(100,200) = (110,210); Y 210 wraps to the north neighbour
+        // 0xC95C (lbY 0x5B → 0x5C), local Y = 210−192 = 18; cell (lx=4, ly=0) → low 0x21.
+        Assert.Equal(0xC95C0021u, body.CellPosition.ObjCellId);
+        Assert.Equal(new Vector3(110f, 18f, 12f), body.CellPosition.Frame.Origin);
+
+        // And the carried (cell, local) is already canonical — re-deriving does not march.
+        uint cell = body.CellPosition.ObjCellId;
+        var local = body.CellPosition.Frame.Origin;
+        Assert.True(LandDefs.AdjustToOutside(ref cell, ref local));
+        Assert.Equal(body.CellPosition.ObjCellId, cell);
+    }
 }