Implement Slice 7 reversible tokenization and restoration

[sidmohan0]

Summary

• canonize the Slice 7 reversible-token contract and versioned DFTOKENv1 envelope
• implement batched, scope-bound tokenization and atomic restoration in the monolithic Rust core
• expose full asynchronous provider flows in Python and Node, with explicit unsupported behavior in browser WASM
• add cross-runtime coverage for Unicode ranges, fresh tokens, authorization failures, malformed/tampered envelopes, provider response validation, and nesting behavior

Verification

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features (65 core tests)
• installed Python package test
• installed Node package test
• browser WASM package test

Scope

No built-in AWS, Google, database, vault, or cryptographic token provider is included; providers remain application-owned behind the canonical async boundary.