feat: implement Slice Three masking and removal

[sidmohan0]

Summary

• canonize the Slice Three transformation configuration contract
• add full and partial Unicode code-point masking with validated custom characters
• add parameterless exact removal with zero-length deletion ranges
• expose discriminated transformation configuration through Python, Node.js, and WASM
• update installed-package contract tests and user documentation

Verification

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