fix(builtins): enforce regex size limits in sed, grep, and awk

[chaliy]

Summary

• Add build_regex() / build_regex_opts() helpers in search_common.rs enforcing 1MB size and DFA limits
• Replace all user-facing Regex::new() and RegexBuilder::new() calls in sed (3), grep (1), awk (8) with bounded helpers
• Internal constant patterns left as-is (safe, known-small)

Test plan

• Grep rejects huge regex pattern (50K alternations)
• Sed rejects huge regex pattern
• AWK rejects huge regex in match()
• Normal regex patterns still compile and work
• All existing spec tests pass

Closes #984