[INJECT-04] Implement folding, keyword classification, and fingerprints

Objective

Match libinjection folding, five-token fingerprint construction, keyword/operator lookup, whitelist and blacklist decisions, reason codes, and tri-state error semantics.

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: INJECT-00

Blocked by

• INJECT-03

Blocks

• INJECT-05

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.

Initial folding/classification slice: 239232e adds a bounded libinjection-shaped fingerprint and deterministic evidence-bearing decisions. CI passed: https://github.com/zig-utils/zig-injection/actions/runs/29892895301. The issue remains open pending the complete upstream keyword/fingerprint tables and differential corpus.