fix(physics): #90 — sphere-overlap cell stickiness at doorway threshold

ResolveCellId's indoor-seed fall-through was point-only: when the indoor
BSP push-back moved the foot-sphere CENTER a few cm outside the indoor
CellBSP volume, the resolver flipped CellId back to outdoor. Next tick
re-promoted via CheckBuildingTransit. The ping-pong caused most ticks
to be classified outdoor, bypassing indoor BSP wall checks entirely
and producing the user-reported "walls walk through everywhere in the
inn" symptom.

Fix: port retail's BSPTREE::sphere_intersects_cell_bsp
(acclient_2013_pseudo_c.txt:323267 → BSPNODE variant at :325546) as
BSPQuery.SphereIntersectsCellBsp(node, center, radius). Replace the
point-only check at PhysicsEngine.ResolveCellId:285 with the radius-
aware overlap test. Player stays classified indoor as long as ANY
part of the foot-sphere still overlaps the indoor cell volume; only
flips to outdoor when the sphere is FULLY outside.

Retail uses a 0.01 m epsilon on the radius (acclient :325551); ported
verbatim. 8 new unit tests cover null/leaf/inside/on-plane/straddling/
fully-outside/tangent-boundary cases plus a regression-anchor test
that proves the old PointInsideCellBsp would have returned false for
the same straddling input.

1147 + 8 baseline maintained (was 1139 + 8 before #90 fix). Closes #90.
A4 multi-cell iteration (shipped earlier today) should now actually
exercise in production since the player can stably remain in indoor
cells.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/BSPQuery.cs

@@ -962,6 +962,51 @@ public static class BSPQuery
         return false;
     }
 
+    /// <summary>
+    /// Issue #90 fix (2026-05-20). Radius-aware sibling of
+    /// <see cref="PointInsideCellBsp"/>. Returns true if a sphere centered
+    /// at <paramref name="center"/> with radius <paramref name="radius"/>
+    /// overlaps the cell volume — i.e., the sphere is fully inside OR
+    /// straddles any splitting plane. Returns false only when the sphere
+    /// is FULLY outside (behind some splitting plane by more than its
+    /// radius).
+    ///
+    /// <para>
+    /// Used by <see cref="PhysicsEngine.ResolveCellId"/> to avoid
+    /// ping-ponging the CellId back to outdoor on each indoor BSP
+    /// push-back: when the indoor BSP pushes the foot-sphere back by
+    /// a few cm, the sphere CENTER may briefly leave the indoor
+    /// CellBSP volume, but the sphere itself still OVERLAPS the cell.
+    /// The point-only test would flip CellId to outdoor; this overlap
+    /// test keeps the player classified as indoor as long as any part
+    /// of the sphere is still inside the cell.
+    /// </para>
+    ///
+    /// <para>
+    /// Retail oracle: <c>BSPTREE::sphere_intersects_cell_bsp</c> at
+    /// <c>acclient_2013_pseudo_c.txt:323267</c> →
+    /// <c>BSPNODE::sphere_intersects_cell_bsp</c> at
+    /// <c>:325546</c>. Retail adds a 0.01f epsilon to the radius before
+    /// the threshold compare; we match that.
+    /// </para>
+    /// </summary>
+    public static bool SphereIntersectsCellBsp(CellBSPNode? node, Vector3 center, float radius)
+    {
+        if (node is null) return true;
+        if (node.Type == BSPNodeType.Leaf) return true;
+
+        float dist = Vector3.Dot(node.SplittingPlane.Normal, center) + node.SplittingPlane.D;
+        float rad  = radius + 0.01f;  // retail's epsilon at :325551
+
+        // Behind splitting plane by more than radius → sphere fully outside.
+        if (dist < -rad) return false;
+
+        // Otherwise (front, on-plane, or straddling within radius) → follow positive child.
+        return node.PosNode is not null
+            ? SphereIntersectsCellBsp(node.PosNode, center, radius)
+            : true;
+    }
+
     // =========================================================================
     // BSP TREE-LEVEL HELPERS
     //