Implements utils/meshcore.py with all dataclasses (MeshcoreMessage,
MeshcoreNode, MeshcoreContact, MeshcoreTelemetry, MeshcoreTraceroute),
connection configs (SerialConfig, TCPConfig, BLEConfig), ConnectionState
enum, serial port discovery, and the MeshcoreClient singleton skeleton.
Adds tests/test_meshcore_client.py covering all dataclasses, availability
check, and state enum (8/8 tests passing).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>