Grows part-to-part spacing of a selected group with the work area and
non-selected parts as hard boundaries. Doubling + bisection search over
the target spacing; Gauss-Seidel straight-line relaxation with anchor
mover policy (later-index selected part moves, first selection never).
Overlapping input is separated along penetration MTVs instead of being
rejected. Cancel/failure never mutates part positions. Clearance gains
BoundaryDistance for ring-pair gaps (part-in-cutout legality).
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