Refactors FindCellList to delegate to a private helper
(BuildCellSetAndPickContaining) that returns BOTH the containing cell
id AND the full candidate HashSet. Public surface gains a new
FindCellSet overload; existing FindCellList behavior is unchanged.
Used by the upcoming Transition.CheckOtherCells (Phase A4) to iterate
every cell the sphere overlaps for per-cell BSP collision. Mirrors
retail's CObjCell::find_cell_list filling both cell_array AND var_4c
at acclient_2013_pseudo_c.txt:272725.
Three new unit tests cover sphere-fully-inside-primary,
sphere-straddling-portal, and outdoor-seed-neighbour-landcells cases.
Spec: docs/superpowers/specs/2026-05-20-phase-a4-multi-cell-bsp-design.md
Plan: docs/superpowers/plans/2026-05-20-phase-a4-multi-cell-bsp.md
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
