Add a from-scratch regex engine and wire it into the JS runtime: - regex.rs: Pattern parser (character classes, quantifiers, anchors, groups, alternation, backreferences, lookahead, escape sequences) and backtracking matcher with proper continuation-based sequence matching for greedy/lazy quantifiers. - RegExp constructor and prototype methods (test, exec, toString) with support for g/i/m/s/u/y flags, lastIndex tracking, and named capture groups. - Compiler emits RegExp(pattern, flags) calls for regex literals. - String.prototype gains match, matchAll, search methods; replace, replaceAll, split updated to handle RegExp arguments with capture group substitution ($1, $&, etc.). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>