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>

src/AcDream.Core/Physics/PhysicsBody.cs

@@ -91,8 +91,73 @@ public sealed class PhysicsBody
     // ── struct fields ──────────────────────────────────────────────────────
     // Offsets from acclient_function_map.md §PhysicsObj Struct Layout.
 
-    /// <summary>World-space position (no offset in struct — frame origin).</summary>
-    public Vector3 Position { get; set; }
+    private Vector3 _position;
+
+    /// <summary>World-space position (frame origin). #145: the setter mirrors its
+    /// delta into <see cref="CellPosition"/> so the cell-relative frame stays exact
+    /// through integration + the resolve-apply (both write here). Placement uses
+    /// <see cref="SnapToCell"/> instead (no delta).</summary>
+    public Vector3 Position
+    {
+        get => _position;
+        set
+        {
+            Vector3 delta = value - _position;
+            _position = value;
+            SyncCellPositionDelta(delta);
+        }
+    }
+
+    /// <summary>
+    /// Cell-relative position (retail Position): the (cell, local∈[0,192)) pair.
+    /// #145 Slice 2 — rides alongside <see cref="Position"/>, becomes authoritative
+    /// in later slices. Default (ObjCellId==0) until <see cref="SnapToCell"/> seeds it.
+    /// </summary>
+    public Position CellPosition { get; private set; }
+
+    /// <summary>
+    /// Placement: set the world position AND seed <see cref="CellPosition"/> from the
+    /// wire's (cell, local) directly — NO streaming center, NO delta. Stores the wire
+    /// values verbatim; the caller has already decoded the wire into a consistent
+    /// (cellId, cellLocal) pair. Caller (Slice 2b) supplies the wire (cell, localX/Y/Z)
+    /// from the inbound position update / teleport arrival.
+    /// </summary>
+    public void SnapToCell(uint cellId, Vector3 worldPos, Vector3 cellLocal)
+    {
+        _position = worldPos;
+        CellPosition = new Position(cellId, new CellFrame(cellLocal, Orientation));
+    }
+
+    // Mirror a world-position translation into the cell-relative frame. Velocity is
+    // frame-invariant under translation, so the same delta applies to the local origin;
+    // AdjustToOutside re-wraps into [0,192) and bumps the cell on a landblock crossing.
+    // Only for a SEEDED OUTDOOR cell (player). Unseeded bodies (ObjCellId==0, e.g. remote
+    // entities) and indoor cells are left untouched here.
+    // We only invoke AdjustToOutside when the local coordinate leaves [0,192) — i.e. on
+    // an actual landblock-boundary crossing. Within the same landblock the cell id is left
+    // unchanged (the wire seed is authoritative for which cell index we occupy).
+    private void SyncCellPositionDelta(Vector3 delta)
+    {
+        uint cell = CellPosition.ObjCellId;
+        if ((cell & 0xFFFFu) is not (>= 1u and <= 0x40u))
+            return;
+        Vector3 local = CellPosition.Frame.Origin + delta;
+        // Only call AdjustToOutside on a landblock-boundary crossing (local X or Y
+        // leaves [0, BlockLength)). Within the block the cell index is kept as-is.
+        if (local.X < 0f || local.X >= LandDefs.BlockLength ||
+            local.Y < 0f || local.Y >= LandDefs.BlockLength)
+        {
+            uint adjusted = cell;
+            if (LandDefs.AdjustToOutside(ref adjusted, ref local))
+                CellPosition = new Position(adjusted, new CellFrame(local, CellPosition.Frame.Orientation));
+            // else: map-edge (AdjustToOutside failed) — leave CellPosition unchanged.
+            // Slice 3 owns proper map-edge membership; this slice is behaviour-neutral.
+        }
+        else
+        {
+            CellPosition = new Position(cell, new CellFrame(local, CellPosition.Frame.Orientation));
+        }
+    }
 
     /// <summary>Orientation quaternion (struct offsets 0x60–0x80 column matrix).</summary>
     public Quaternion Orientation { get; set; } = Quaternion.Identity;