Implement Slice One finding and scan contract

[sidmohan0]

Summary

• add the accepted Slice Zero privacy-core contract documentation
• replace the legacy entity result with the canonical Finding and explicit TextRange types
• expose UTF-8 byte and Unicode code-point ranges with optional confidence and detector provenance
• carry the contract through the Python, Node, and browser/WASM bindings
• preserve existing detector fixture coverage while adding Unicode range conformance checks

Verification

• cargo test --workspace (18 core tests passed)
• installed Python wheel fixture and Finding-contract test
• packed Node package fixture, runtime, and TypeScript tests
• packed browser/WASM package fixture, runtime, and TypeScript tests
• git diff --check

Notes

• Core implementation and unit tests remain in the canonical monolithic crates/core/src/lib.rs.
• Node package verification ran successfully under Node 25, with the expected engine warning because the package currently declares Node 24.x.