feat: add Android USB serial port support via USB Host API

- Add USB Host permissions and device filter to AndroidManifest.xml
- Create UsbSerialPlugin Kotlin plugin for USB Host API (enumerate, permission, open devices)
- Add serial_connect_fd command for Android to accept USB file descriptors
- Create RawFdStream wrapper for async I/O on raw file descriptors
- Make run_serial_with_poll generic over AsyncRead+AsyncWrite
- Register UsbSerialPlugin in MainActivity

src-tauri/Cargo.toml

@@ -53,6 +53,8 @@ uuid = { version = "1", features = ["v4", "serde"] }
 rand = "0.8"
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls"] }
 ndarray = { version = "0.15", optional = true }
+[target.'cfg(target_os = "android")'.dependencies]
+libc = "0.2"
 
 [target.'cfg(not(any(target_os = "android", target_os = "ios")))'.dependencies]
 tauri-plugin-updater = "2"