feat(domain): implement RelayController trait and error handling
Add RelayController async trait (T030) defining interface for Modbus
relay operations with methods for read/write state, bulk operations,
connection checks, and firmware queries.
Implement ControllerError enum (T031) with variants for connection
failures, timeouts, Modbus exceptions, and invalid relay IDs.
Provide MockRelayController (T029) in-memory implementation using
Arc<Mutex<HashMap>> for thread-safe state storage with timeout
simulation for error handling tests.
Add RelayLabelRepository trait abstraction.
Ref: T029 T030 T031 (specs/001-modbus-relay-control/tasks.md)
phundrak
e8e6a1e702
