Add portable file streaming APIs

[AbassHammed]

Summary

• add portable frontend byte stream APIs for desktop, Android, and iOS
• add Rust-side filesystem API with app.vnidrop_fs() and stream reader/writer handles
• add a Rust-side example app that keeps filesystem operations in backend commands
• document stream usage and add regression/type tests

Checks

• npm run check
• npm run build in examples/tauri-rust-side-app
• cargo check in examples/tauri-rust-side-app/src-tauri
• cargo check --target aarch64-apple-ios-sim --all-features --features tauri/wry
• npm run test:ios
• npm run test:android:jvm