feat(errors): best-in-class arity messages — name + expected + got (ERRORS.md P6)Verified

Anthony Green2026-06-13 21:15:33 -0400 38aa5285a64724e075ec98637d5d980f56fe0f05

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