`modules/radio_link.py` now holds connecting and reconnecting the transport, the host->radio command serializer and `radio_session`, zombie state and the periodic health probe: 20 methods moved verbatim, so every name is still on `MeshCoreBot`. `_serialize_command_frames` and `_radio_session_held` moved with them and are re-exported from `modules.core`; the serializer's `bot` annotation is now the mixin. `core.py` keeps `import meshcore` because tests patch `modules.core.meshcore.MeshCore`. core.py: 2,164 → 1,599 lines.
