Add Rust directory filesystem facade

[AbassHammed]

Summary

• add Rust-facing directory targets, entry metadata, directory listing, recursive walking, relative directory creation, and relative file writer APIs on VnidropFs
• wire Android and iOS provider-specific directory operations behind the shared Rust facade
• add iOS native resolve helpers so relative writes reuse existing files and create only when requested
• extend backend Rust tests for directory walking, relative writes, metadata, and unsafe relative path rejection

Validation

• cargo test
• cargo check --all-features
• npm run test -- --run
• npm run test:types
• npm run build
• npm run test:ios