Implements T025-T027 from TDD workflow (red-green-refactor):
- T025 (red): Tests for ModbusAddress with From<RelayId> conversion
- T026 (green): ModbusAddress newtype (#[repr(transparent)]) with offset mapping
- T027 (red+green): HealthStatus enum with state transitions
ModbusAddress wraps u16 and converts user-facing relay IDs (1-8) to
Modbus addresses (0-7) at the domain boundary. HealthStatus tracks
relay health with Healthy, Degraded, and Unhealthy states supporting
error tracking and recovery monitoring.
Ref: T025, T026, T027 (specs/001-modbus-relay-control)
Lucien Cartier-Tilet
7e10823714
