regex surface API

[hackwaly]

Summary

This PR introduces a public Regex surface API for string processing, hardens Unicode/surrogate correctness in regex matching, and aligns substring APIs with panic-on-invalid-index behavior.

What changed

• Exposed Regex as a public type and added pattern-based construction with lazy internal compilation.
• Added higher-level Regex capabilities for:
  • literal regex construction
  • regex composition (sequence and alternation)
  • quantifier-based repetition
  • iterative matching, regex-based splitting, and callback-based replacement
• Expanded match result helpers for full match content, group access, named-group access, and before/after slices.
• Re-exported Regex from prelude for easier usage.
• Added user-facing documentation and comprehensive tests for API behavior and edge cases.

Unicode and boundary fixes

• Character-class parsing now clips positive ranges to valid profile code-point sets.
• Unicode profile handling excludes surrogate holes to avoid half-surrogate matches.
• Word-boundary handling now distinguishes start/end non-word categories around surrogate pairs, fixing incorrect non-word-boundary behavior in those positions.

Behavioral change

• String::sub and StringView::sub now panic on invalid indices instead of raising a view-creation error.
• The corresponding view-creation error type is removed from the public surface.
• Related call sites, tests, and changelog entries are updated.

Validation

• Added and updated extensive tests covering compile/execute paths, split/find/replace behavior, zero-width matches, capture and named-capture access, and surrogate-pair edge cases.

[coveralls]

Pull Request Test Coverage Report for Build 2643

Details

• 83 of 91 (91.21%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 96.711%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
bytes/regex.mbt	5	6	83.33%
string/regex.mbt	6	7	85.71%
bytes/regex_methods.mbt	29	32	90.63%
string/regex_methods.mbt	33	36	91.67%

Totals
Change from base Build 2631:	0.7%
Covered Lines:	12086
Relevant Lines:	12497

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💛 - Coveralls