feat: native macOS CoreBluetooth BLE plugin (bypass flutter_blue_plus)

- Add BLEPlugin.swift — native CoreBluetooth Central via platform channel
  - CBConnectPeripheralOptionNotifyOn* connection options (matching original)
  - Service/characteristic discovery + notification subscription
  - 15s connection timeout with manual cancellation
- Add ble_native_channel.dart — Dart MethodChannel/EventChannel interface
- Register plugin in AppDelegate.swift
- Add to Xcode project (project.pbxproj)
- Rewrite BLEMeshService with platform-conditional paths:
  - macOS: native CoreBluetooth via BLENativeChannel
  - iOS/Android: flutter_blue_plus (unchanged)
- Add test override for native channel detection

Author: ebwood