Art-Net RDM/TOD helpers for the impl architecture. Maintains a
lightweight Table-of-Devices snapshot and subscriber registry so the logic
coordinator can focus on orchestration.

ArtRdm and ArtRdmSub transport validation and helpers per Art-Net 4 spec.

Art-Net 4 §ArtRdm:
- Transport all non-discovery RDM messages
- Unicast only (broadcast no longer allowed in Art-Net 4)
- Command field 0x00 = ArProcess (process RDM packet)
- Validate command class in RDM payload (byte 20)

Art-Net 4 §ArtRdmSub:
- Compressed sub-device data transfer
- Get (0x20): 0 data bytes
- Set (0x30): SubCount * 2 data bytes
- GetResponse (0x21): SubCount * 2 data bytes
- SetResponse (0x31): 0 data bytes

This module provides pure functions for:
- RDM command class validation
- ArtRdmSub packet validation
- Sub-device range and entry helpers
- Payload normalization for outbound commands