phase 0 of p2p rollback netcode. no network yet; the whole job here is determinism. the sim keeps every gameplay value in one Int32Array, so a snapshot is a slice and a checksum is a walk. integers only — the spec calls sin/cos/pow "implementation-approximated" and lets engines choose their own algorithm, so one Math.sin would desync chrome against safari. `rollback.mjs` is ggpo's synctest: every frame, rewind N frames and replay them, and the checksums must agree. green at distances 1, 2, 4 and 8. two players share a qwerty — wasd+fgh and arrows+jkl. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>