[codex] Map PLC canonical tags for Hub UNS

[Mikecranesync]

Summary

• Adds TagSnapshot support for ModbusTagSource.
• Maps Micro820 snapshots and raw conveyor/Stardust names to canonical Hub tags.
• Adds regression coverage for the Task 4 canonical tag contract.

Validation

• PYTHONPATH=src pytest tests/unit/test_modbus_tag_source.py -q
• PYTHONPATH=src pytest tests/unit -q

Notes

• This PR prepares canonical PLC output; relay publishing is planned as the next slice.