Add serial capability for Bluetooth Classic and USB OTG

[mithun50]

Summary

Closes #21

• Adds a unified serial capability with Bluetooth Classic (RFCOMM/SPP) and USB serial (CDC-ACM, FTDI, CH340, CP210x) support
• AI agents can now scan, connect, write, read, disconnect, and list serial devices via the Node protocol
• Smart permission gating: Bluetooth permissions only requested for BT transport, USB needs no runtime permissions

Commands

Command	Description
serial.scan	Discover USB devices or paired Bluetooth devices
serial.connect	Open a persistent serial connection
serial.write	Send data (utf8 or base64 encoded)
serial.read	Read buffered data with timeout and delimiter support
serial.disconnect	Close one or all connections
serial.list	List active connections with buffer sizes

Changes

• New: serial_capability.dart — 530-line capability handler with 64KB read buffers
• Modified: pubspec.yaml — added flutter_serial_communication and flutter_bluetooth_classic_serial
• Modified: AndroidManifest.xml — Bluetooth + USB host permissions
• Modified: node_provider.dart — registered serial capability
• Modified: node_screen.dart — added Bluetooth Serial and USB Serial UI tiles

Test plan

• serial.scan transport:"usb" with Arduino connected via USB OTG → device appears
• serial.connect → returns connectionId
• serial.write + serial.read → round-trip data verified
• serial.scan transport:"bluetooth" with paired Micro:bit → device appears
• BT connect + write/read cycle
• serial.disconnect connectionId:"all" + serial.list → empty
• Deny BT permission → PERMISSION_DENIED error returned

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