feat(domain): add ModbusAddress type and HealthStatus enum

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)

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

@@ -260,19 +260,19 @@
   - **File**: src/domain/relay.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T025** [US1] [TDD] Write tests for ModbusAddress type
+- [x] **T025** [US1] [TDD] Write tests for ModbusAddress type
   - Test: ModbusAddress::from(RelayId(1)) → ModbusAddress(0)
   - Test: ModbusAddress::from(RelayId(8)) → ModbusAddress(7)
   - **File**: src/domain/modbus.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T026** [US1] [TDD] Implement ModbusAddress type with From<RelayId>
+- [x] **T026** [US1] [TDD] Implement ModbusAddress type with From<RelayId>
   - #[repr(transparent)] newtype wrapping u16
   - Implement From<RelayId> with offset: user 1-8 → Modbus 0-7
   - **File**: src/domain/modbus.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T027** [US3] [TDD] Write tests and implement HealthStatus enum
+- [x] **T027** [US3] [TDD] Write tests and implement HealthStatus enum
   - Enum: Healthy, Degraded { consecutive_errors: u32 }, Unhealthy { reason: String }
   - Test transitions between states
   - **File**: src/domain/health.rs