Breaking behavior change in lookahead/negation assertions (both now
return `true` instead of `undefined` so matches can be distinguished
from failures when the assertion is marked optional, e.g.
`(&Pattern)?`), plus the accumulated Dev/docs/internal work since
0.8.20 went out on npm.

See CHANGELOG.md for the full list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>