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)

specs/001-modbus-relay-control/tasks.md

@@ -293,13 +293,13 @@
   - **File**: src/infrastructure/modbus/mock_controller.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T029** [P] [US1] [TDD] Implement MockRelayController
+- [x] **T029** [P] [US1] [TDD] Implement MockRelayController
   - Struct with Arc<Mutex<HashMap<RelayId, RelayState>>>
   - Implement RelayController trait with in-memory state
   - **File**: src/infrastructure/modbus/mock_controller.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T030** [US1] [TDD] Define RelayController trait
+- [x] **T030** [US1] [TDD] Define RelayController trait
   - async fn read_state(&self, id: RelayId) → Result<RelayState, ControllerError>
   - async fn write_state(&self, id: RelayId, state: RelayState) → Result<(), ControllerError>
   - async fn read_all(&self) → Result<Vec<(RelayId, RelayState)>, ControllerError>
@@ -307,7 +307,7 @@
   - **File**: src/infrastructure/modbus/controller.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T031** [P] [US1] [TDD] Define ControllerError enum
+- [x] **T031** [P] [US1] [TDD] Define ControllerError enum
   - Variants: ConnectionError(String), Timeout(u64), ModbusException(String), InvalidRelayId(u8)
   - Implement std::error::Error, Display, Debug
   - Use thiserror derive macros