[REGEX-02] Close syntax, capture, Unicode, and matching-semantic gaps

Objective

Implement required inline modifiers, Unicode properties, named groups and backreferences, atomic and possessive constructs, lookarounds, conditionals, escapes, anchors, and capture semantics found by the differential suite.

Deliverables

• Implement the complete scoped behavior without Git submodules.
• Keep public APIs explicit about ownership, resource limits, and errors.
• Add focused unit, integration, compatibility, fuzz, and benchmark coverage as applicable.
• Update documentation and machine-readable compatibility status.

Acceptance criteria

• The scoped implementation and tests are merged on main.
• Zig formatting, focused tests, and the repository CI suite pass.
• No unexplained compatibility skips, silent fallbacks, leaks, or unbounded hostile-input paths remain.
• Benchmarks or security evidence is attached when this affects a hot or untrusted path.

Relationships

• Parent: REGEX-00

Blocked by

• REGEX-01

Blocks

• REGEX-03
• REGEX-04

Baselines

• Zig 0.17-dev resolved and locked through Pantry.
• ModSecurity 3.0.16, Coraza 3.7.0, CRS 4.28.0, and libinjection 4.0.0 where applicable.