refactor: migrate from rusb to nusb for USB transport

Replace rusb (libusb bindings) with nusb (pure Rust USB library):

- Remove rusb dependency, add nusb and futures-lite
- Simplify USB code: DeviceHandle -> Interface
- Use async bulk_out/bulk_in APIs with block_on for blocking
- Interface automatically released on drop (no manual release needed)
- Keep Windows CH375DLL support unchanged

Benefits:
- Pure Rust, no C library dependency
- Easier cross-compilation (no libusb to compile)
- Cleaner API

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: andelf