Times Collision.HasOverlap over the box-overlapping neighbour pairs of two
FillLinear grids, and the FillHelpers.HasOverlappingParts grid checks
themselves. Opt-in via OPENNEST_RUN_FILL_PERF=1 (Category=FillPerformance).
Uses only APIs that predate the overlap-check work, so the same file can be
copied into a before tree for same-harness comparisons.
