box3d: README — document opt-in broad-phase hybrid, annotate washer

Broad-phase-hybrid subsection (why washer loses, the batch design, the
determinism proof, the single-thread win / multi-thread floor tradeoff,
and the default-off rationale). Washer row in the single-thread rapier
table annotated with the opt-in number (~17.7s, beats C, ~5% behind
rapier → box3d ahead-or-even on all nine single-threaded).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>