fix(physics): S1B — indoor cell membership admits on the part BOX, as retail does (#335, AP-159 narrowed)

CellTransit.FindTransitCellsBox ports CEnvCell::find_transit_cells'
part-array overload @0x0052cae0 line-for-line: per-portal x per-part
order, the sphere cheap-reject at F_EPSILON+radius, the box admit whose
"Straddle or crossing-side" rule is exactly retail's `eax != side` under
the PDB Sidedness enum, leads-outside placed AFTER the admit, the
unconditional unloaded-neighbour hint without the sphere overload's
re-test, the destination box_intersects_cell gate with its deliberate
no-break, and add_all_outside_cells after the loop. The box-vs-cell BSP
traversal lands in BOTH representations behind the flat-authoritative
dispatcher with a graph referee whose 20,000 installed comparisons are
pinned by assertion (review F5), zero mismatch.

Dual Opus review: PASS on both lenses. The mandatory D0 pseudocode pass
caught that the contract's own supplementary note misattributed the box
block to the sphere overload — it belongs to a SECOND
check_building_transit overload @0x0052c680, whose portal-side
convention is INVERTED and whose admit differs; the pseudocode doc now
records that trap plus two byte confirmations made at review:
which_side @0x00444720 is strictly > eps for POSITIVE, and
intersect_box's in-plane early exit returns CROSSING(3)
(jp @0x005aa1bc -> mov eax,3), settling review items b1/b2 for the
future bridge porter. The bridge itself stays unported as AP-159's
explicit remainder.

The review also retired #335's severity premise honestly: "over-
inclusive only, never a missed one" is wrong at production shape ratios,
where the box (whole-vertex AABB) legitimately exceeds the sphere
(physics-polygon root sphere). Measured, both populations: rigged
(box << sphere) — 1,520 placements, 978 cells removed, 0 added;
production-ratio (box >= sphere) — 950 placements, 20 removed, 1 ADDED
through the loaded-neighbour gate, which is retail's direction, not a
defect. The no-op guard (review F4) asserts removal is nonzero so an
unwired admit cannot pass silently.

Process note: the implementer authored against this session's worktree
at bec5c69d, 25 commits stale — the recorded worktree-base class. All
six files were byte-identical between bases, the diff transplanted
losslessly, and every verdict-bearing run (referee, direction sweeps,
this clean-room) was re-executed on current main. S2's uncommitted
phase-1 edits were stashed for this landing so the suite verdicts
exactly one changeset.

Also untracks 341-slope-capture.jsonl (an accidental add) and
gitignores it.

Clean-room suite: 11,248 passed / 6 skipped / 0 failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/AcDream.Core/Physics/FlatBspQuery.cs

@@ -476,6 +476,51 @@ internal static class FlatBspQuery
             : true;
     }
 
+    /// <summary>
+    /// Flat port of retail <c>BSPNODE::box_intersects_cell_bsp</c>
+    /// (0x0053C880). AP-159 / #335 (2026-08-07). Shares
+    /// <see cref="BSPQuery.ClassifyBox"/> for the box-vs-plane classification
+    /// math, exactly like this file's polygon methods share
+    /// <see cref="BSPQuery.PolygonHitsSpherePrecise"/>. See
+    /// docs/research/2026-08-07-ap159-pseudocode.md §3.
+    /// </summary>
+    public static bool BoxIntersectsCellBsp(
+        FlatCellContainmentBsp tree,
+        Vector3 min,
+        Vector3 max)
+    {
+        ArgumentNullException.ThrowIfNull(tree);
+        return BoxIntersectsCellBsp(tree, tree.RootIndex, min, max);
+    }
+
+    private static bool BoxIntersectsCellBsp(
+        FlatCellContainmentBsp tree,
+        int nodeIndex,
+        Vector3 min,
+        Vector3 max)
+    {
+        if (nodeIndex < 0)
+            return true;
+
+        FlatCellBspNode node = tree.Nodes[nodeIndex];
+        if (node.Type == BSPNodeType.Leaf)
+            return true;
+
+        if (BSPQuery.ClassifyBox(node.SplittingPlane, min, max) ==
+            BSPQuery.PlaneSide.Negative)
+        {
+            return false;
+        }
+
+        return node.PositiveChildIndex >= 0
+            ? BoxIntersectsCellBsp(
+                tree,
+                node.PositiveChildIndex,
+                min,
+                max)
+            : true;
+    }
+
     /// <summary>
     /// Flat static sphere/polygon overlap shadow query.
     /// </summary>