chore(deps): upgrade dev dependencies to latest majors (#825)

## Summary

- `@size-limit/file` ^11 → ^12
- `size-limit` ^11 → ^12
- `big.js` 6.2.2 → 7.0.1
- `fast-check` ^3 → ^4
- `lint-staged` ^15 → ^16

All tests (829), type checks, and linting pass without any code changes.
`@types/big.js` stays at 6.2.2 since big.js v7 doesn't ship its own
types and the existing types remain compatible.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: sarahdayan