fix(physics #145): Slice 2a — canonicalize outdoor seed + re-derive cell index every tick

The wire local is LANDBLOCK-relative [0,192); the cell low word = floor(local/24),
so a consistent (cell,local) pair must keep them in lockstep. Two retail-faithful
corrections vs the first pass:
 - SnapToCell canonicalizes the OUTDOOR seed via AdjustToOutside (retail
   SetPositionInternal/adjust_to_outside @0x00504A40 — the #107 'never trust a
   server (cell,pos) pair' protection). Indoor seeds stay verbatim (BSP-validated).
 - SyncCellPositionDelta calls AdjustToOutside on EVERY delta, not just on 192 m
   crossings, so intra-landblock 24 m cell-index changes track (needed by Slice 3
   membership). Idempotent within a cell.
Tests rewritten to verify both (the earlier test paired an inconsistent cell+local).
Core 1527 passed / 0 failed.

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

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

@@ -4,46 +4,63 @@ using Xunit;
 
 namespace AcDream.Core.Tests.Physics;
 
+// #145 Slice 2a — PhysicsBody carries a cell-relative CellPosition that rides alongside
+// the world Vector3 Position. The wire's local is LANDBLOCK-relative [0,192); the cell id
+// low word (which 24 m cell) = floor(local/24), so a consistent pair has
+// cell.lowword == cellIndex(local). Placement canonicalizes the outdoor seed (retail
+// SetPositionInternal/adjust_to_outside); motion mirrors the world delta into the local
+// and re-derives the cell every tick via AdjustToOutside.
 public class PhysicsBodyCellSyncTests
 {
     [Fact]
-    public void SnapToCell_SeedsCellPosition_AndWorldPosition()
+    public void SnapToCell_ConsistentOutdoorPair_SeedsAsGiven()
     {
         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);
+        // cell 0x23 = (lx=4, ly=2); local (100,50) is in that cell (floor(100/24)=4, floor(50/24)=2).
+        body.SnapToCell(0xC95B0023u, worldPos: new Vector3(1000f, 2000f, 12f), cellLocal: new Vector3(100f, 50f, 12f));
+        Assert.Equal(0xC95B0023u, body.CellPosition.ObjCellId);
+        Assert.Equal(new Vector3(100f, 50f, 12f), body.CellPosition.Frame.Origin);
         Assert.Equal(new Vector3(1000f, 2000f, 12f), body.Position);
     }
 
     [Fact]
-    public void PositionDelta_MirrorsIntoCellPosition_WithinLandblock()
+    public void SnapToCell_InconsistentOutdoorPair_CanonicalizesCellFromLocal()
     {
+        // #107 protection: retail re-derives the cell from the position on placement.
+        // (cell 0x01, local in cell 0x23) is fixed to 0x23.
         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
+        Assert.Equal(0xC95B0023u, body.CellPosition.ObjCellId);
+        Assert.Equal(new Vector3(100f, 50f, 12f), body.CellPosition.Frame.Origin);
+    }
+
+    [Fact]
+    public void PositionDelta_WithinLandblock_UpdatesCellIndex()
+    {
+        var body = new PhysicsBody();
+        body.SnapToCell(0xC95B0023u, new Vector3(1000f, 2000f, 12f), new Vector3(100f, 50f, 12f));
+        body.Position += new Vector3(0f, 24f, 0f);   // local (100,74) → cell (4,3)
+        Assert.Equal(new Vector3(100f, 74f, 12f), body.CellPosition.Frame.Origin);
+        // low word for (lx=4, ly=3): (ly&7) + ((lx&7)<<3) + 1 = 3 + 32 + 1 = 0x24
+        Assert.Equal(0xC95B0024u, body.CellPosition.ObjCellId);
+        Assert.Equal(new Vector3(1000f, 2024f, 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);
+        body.Position += new Vector3(0f, -1f, 0f);   // local Y → -0.7 → south neighbour, ~191.3
         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
+        Assert.Equal(0x5A, lbY);                                       // bumped to south neighbour 0xC95A
+        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.
+        // A body never SnapToCell'd (e.g. a remote entity) gets no synthesized cell.
         var body = new PhysicsBody();
         body.Position += new Vector3(10f, 10f, 0f);
         Assert.Equal(0u, body.CellPosition.ObjCellId);