feat: implement Slice Two redaction

[sidmohan0]

Summary

• add strict explicit-finding transformation and scan-and-transform convenience APIs
• implement deterministic duplicate/overlap resolution and unnumbered entity redaction
• expose the contract through Rust, Python, Node.js, and WASM
• document the accepted Slice Two behavior and mark the roadmap slice complete

Verification

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features
• installed Python wheel contract test
• installed Node package and TypeScript contract tests
• installed WASM package, TypeScript, and browser contract tests