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)

backend/src/domain/health.rs

@@ -0,0 +1,295 @@
+//! Health monitoring domain module.
+//!
+//! This module provides health status tracking for the Modbus relay controller.
+//! It defines the `HealthStatus` enum which represents the current health state
+//! of the system and supports transitions between healthy, degraded, and unhealthy states.
+
+/// Health status of the Modbus relay controller.
+///
+/// Represents the three possible health states of the system:
+/// - `Healthy`: System is operating normally
+/// - `Degraded`: System is experiencing errors but still operational
+/// - `Unhealthy`: System has critical failures
+///
+/// # State Transitions
+///
+/// ```text
+/// Healthy ──(errors)──> Degraded ──(more errors)──> Unhealthy
+///    ^                       |                          |
+///    └──────(recovery)───────┘                          |
+///    └────────────────(recovery)────────────────────────┘
+/// ```
+///
+/// # Examples
+///
+/// ```
+/// use sta::domain::health::HealthStatus;
+///
+/// let status = HealthStatus::Healthy;
+/// assert!(matches!(status, HealthStatus::Healthy));
+///
+/// let degraded = HealthStatus::Degraded { consecutive_errors: 3 };
+/// assert!(matches!(degraded, HealthStatus::Degraded { .. }));
+///
+/// let unhealthy = HealthStatus::Unhealthy {
+///     reason: "Connection timeout".to_string()
+/// };
+/// assert!(matches!(unhealthy, HealthStatus::Unhealthy { .. }));
+/// ```
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum HealthStatus {
+    /// System is operating normally with no errors.
+    Healthy,
+
+    /// System is experiencing errors but still operational.
+    ///
+    /// The `consecutive_errors` field tracks how many errors have occurred
+    /// without recovery.
+    Degraded {
+        /// Number of consecutive errors without recovery.
+        consecutive_errors: u32,
+    },
+
+    /// System has critical failures and is not operational.
+    ///
+    /// The `reason` field provides a human-readable description of the failure.
+    Unhealthy {
+        /// Human-readable description of the failure reason.
+        reason: String,
+    },
+}
+
+impl HealthStatus {
+    /// Creates a new `Healthy` status.
+    #[must_use]
+    pub const fn healthy() -> Self {
+        Self::Healthy
+    }
+
+    /// Creates a new `Degraded` status with the given error count.
+    #[must_use]
+    pub const fn degraded(consecutive_errors: u32) -> Self {
+        Self::Degraded { consecutive_errors }
+    }
+
+    /// Creates a new `Unhealthy` status with the given reason.
+    #[must_use]
+    pub fn unhealthy(reason: impl Into<String>) -> Self {
+        Self::Unhealthy {
+            reason: reason.into(),
+        }
+    }
+
+    /// Returns `true` if the status is `Healthy`.
+    #[must_use]
+    pub const fn is_healthy(&self) -> bool {
+        matches!(self, Self::Healthy)
+    }
+
+    /// Returns `true` if the status is `Degraded`.
+    #[must_use]
+    pub const fn is_degraded(&self) -> bool {
+        matches!(self, Self::Degraded { .. })
+    }
+
+    /// Returns `true` if the status is `Unhealthy`.
+    #[must_use]
+    pub const fn is_unhealthy(&self) -> bool {
+        matches!(self, Self::Unhealthy { .. })
+    }
+
+    /// Transitions to a degraded state by incrementing the error count.
+    ///
+    /// If already degraded, increments the consecutive errors.
+    /// If healthy, transitions to degraded with 1 error.
+    /// If unhealthy, remains unhealthy.
+    #[must_use]
+    pub fn record_error(self) -> Self {
+        match self {
+            Self::Healthy => Self::Degraded { consecutive_errors: 1 },
+            Self::Degraded { consecutive_errors } => {
+                Self::Degraded {
+                    consecutive_errors: consecutive_errors + 1,
+                }
+            }
+            Self::Unhealthy { reason } => Self::Unhealthy { reason },
+        }
+    }
+
+    /// Transitions to a healthy state, resetting all error counts.
+    #[must_use]
+    pub fn record_success(self) -> Self {
+        Self::Healthy
+    }
+
+    /// Transitions to unhealthy state with the given reason.
+    #[must_use]
+    pub fn mark_unhealthy(self, reason: impl Into<String>) -> Self {
+        Self::Unhealthy {
+            reason: reason.into(),
+        }
+    }
+}
+
+impl std::fmt::Display for HealthStatus {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Healthy => write!(f, "Healthy"),
+            Self::Degraded { consecutive_errors } => {
+                write!(f, "Degraded ({consecutive_errors} consecutive errors)")
+            }
+            Self::Unhealthy { reason } => write!(f, "Unhealthy: {reason}"),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_healthy_status_creation() {
+        let status = HealthStatus::healthy();
+        assert!(status.is_healthy());
+        assert!(!status.is_degraded());
+        assert!(!status.is_unhealthy());
+    }
+
+    #[test]
+    fn test_degraded_status_creation() {
+        let status = HealthStatus::degraded(3);
+        assert!(!status.is_healthy());
+        assert!(status.is_degraded());
+        assert!(!status.is_unhealthy());
+        assert_eq!(status, HealthStatus::Degraded { consecutive_errors: 3 });
+    }
+
+    #[test]
+    fn test_unhealthy_status_creation() {
+        let status = HealthStatus::unhealthy("Connection timeout");
+        assert!(!status.is_healthy());
+        assert!(!status.is_degraded());
+        assert!(status.is_unhealthy());
+        assert_eq!(
+            status,
+            HealthStatus::Unhealthy {
+                reason: "Connection timeout".to_string()
+            }
+        );
+    }
+
+    #[test]
+    fn test_transition_healthy_to_degraded() {
+        let status = HealthStatus::healthy();
+        let status = status.record_error();
+        assert!(status.is_degraded());
+        assert_eq!(status, HealthStatus::Degraded { consecutive_errors: 1 });
+    }
+
+    #[test]
+    fn test_transition_degraded_increments_errors() {
+        let status = HealthStatus::degraded(2);
+        let status = status.record_error();
+        assert!(status.is_degraded());
+        assert_eq!(status, HealthStatus::Degraded { consecutive_errors: 3 });
+    }
+
+    #[test]
+    fn test_transition_unhealthy_stays_unhealthy_on_error() {
+        let status = HealthStatus::unhealthy("Device disconnected");
+        let status = status.record_error();
+        assert!(status.is_unhealthy());
+        assert_eq!(
+            status,
+            HealthStatus::Unhealthy {
+                reason: "Device disconnected".to_string()
+            }
+        );
+    }
+
+    #[test]
+    fn test_transition_healthy_to_unhealthy() {
+        let status = HealthStatus::healthy();
+        let status = status.mark_unhealthy("Critical failure");
+        assert!(status.is_unhealthy());
+        assert_eq!(
+            status,
+            HealthStatus::Unhealthy {
+                reason: "Critical failure".to_string()
+            }
+        );
+    }
+
+    #[test]
+    fn test_transition_degraded_to_unhealthy() {
+        let status = HealthStatus::degraded(5);
+        let status = status.mark_unhealthy("Too many errors");
+        assert!(status.is_unhealthy());
+        assert_eq!(
+            status,
+            HealthStatus::Unhealthy {
+                reason: "Too many errors".to_string()
+            }
+        );
+    }
+
+    #[test]
+    fn test_transition_degraded_to_healthy() {
+        let status = HealthStatus::degraded(3);
+        let status = status.record_success();
+        assert!(status.is_healthy());
+    }
+
+    #[test]
+    fn test_transition_unhealthy_to_healthy() {
+        let status = HealthStatus::unhealthy("Device offline");
+        let status = status.record_success();
+        assert!(status.is_healthy());
+    }
+
+    #[test]
+    fn test_display_healthy() {
+        let status = HealthStatus::healthy();
+        assert_eq!(status.to_string(), "Healthy");
+    }
+
+    #[test]
+    fn test_display_degraded() {
+        let status = HealthStatus::degraded(5);
+        assert_eq!(status.to_string(), "Degraded (5 consecutive errors)");
+    }
+
+    #[test]
+    fn test_display_unhealthy() {
+        let status = HealthStatus::unhealthy("Connection lost");
+        assert_eq!(status.to_string(), "Unhealthy: Connection lost");
+    }
+
+    #[test]
+    fn test_multiple_transitions() {
+        // Start healthy
+        let status = HealthStatus::healthy();
+        assert!(status.is_healthy());
+
+        // Record first error -> Degraded with 1 error
+        let status = status.record_error();
+        assert!(status.is_degraded());
+        assert_eq!(status, HealthStatus::Degraded { consecutive_errors: 1 });
+
+        // Record second error -> Degraded with 2 errors
+        let status = status.record_error();
+        assert_eq!(status, HealthStatus::Degraded { consecutive_errors: 2 });
+
+        // Record third error -> Degraded with 3 errors
+        let status = status.record_error();
+        assert_eq!(status, HealthStatus::Degraded { consecutive_errors: 3 });
+
+        // Mark unhealthy -> Unhealthy
+        let status = status.mark_unhealthy("Too many consecutive errors");
+        assert!(status.is_unhealthy());
+
+        // Recover -> Healthy
+        let status = status.record_success();
+        assert!(status.is_healthy());
+    }
+}

backend/src/domain/mod.rs

@@ -36,3 +36,5 @@
 //! - Domain specification: `specs/001-modbus-relay-control/spec.md`
 
 pub mod relay;
+pub mod modbus;
+pub mod health;

backend/src/domain/modbus.rs

@@ -0,0 +1,89 @@
+//! Modbus domain module.
+//!
+//! This module contains Modbus-specific domain types and conversions.
+//! It provides a clean abstraction layer between user-facing relay IDs
+//! and Modbus protocol addresses.
+
+use super::relay::types::RelayId;
+
+/// Modbus address newtype wrapping u16.
+///
+/// Represents a Modbus coil address (0-based indexing).
+/// User-facing relay IDs (1-8) are converted to Modbus addresses (0-7).
+///
+/// # Examples
+///
+/// ```
+/// use sta::domain::modbus::ModbusAddress;
+/// use sta::domain::relay::types::RelayId;
+///
+/// let relay_id = RelayId::new(1).unwrap();
+/// let modbus_addr = ModbusAddress::from(relay_id);
+/// assert_eq!(modbus_addr.as_u16(), 0);
+/// ```
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[repr(transparent)]
+pub struct ModbusAddress(u16);
+
+impl ModbusAddress {
+    /// Returns the inner u16 value.
+    #[must_use]
+    pub const fn as_u16(self) -> u16 {
+        self.0
+    }
+}
+
+/// Converts a user-facing `RelayId` (1-8) to a Modbus address (0-7).
+///
+/// # Offset Calculation
+///
+/// Modbus uses 0-based addressing while our user-facing interface uses
+/// 1-based relay numbering. This conversion applies the -1 offset.
+///
+/// # Examples
+///
+/// ```
+/// use sta::domain::modbus::ModbusAddress;
+/// use sta::domain::relay::types::RelayId;
+///
+/// let relay1 = RelayId::new(1).unwrap();
+/// assert_eq!(ModbusAddress::from(relay1).as_u16(), 0);
+///
+/// let relay8 = RelayId::new(8).unwrap();
+/// assert_eq!(ModbusAddress::from(relay8).as_u16(), 7);
+/// ```
+impl From<RelayId> for ModbusAddress {
+    fn from(relay_id: RelayId) -> Self {
+        // RelayId is guaranteed to be 1..=8 by its validation
+        // Convert to 0..=7 for Modbus addressing
+        Self(u16::from(relay_id.as_u8() - 1))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_relay_id_1_converts_to_modbus_address_0() {
+        let relay_id = RelayId::new(1).unwrap();
+        let modbus_addr = ModbusAddress::from(relay_id);
+        assert_eq!(modbus_addr.as_u16(), 0);
+    }
+
+    #[test]
+    fn test_relay_id_8_converts_to_modbus_address_7() {
+        let relay_id = RelayId::new(8).unwrap();
+        let modbus_addr = ModbusAddress::from(relay_id);
+        assert_eq!(modbus_addr.as_u16(), 7);
+    }
+
+    #[test]
+    fn test_all_relay_ids_convert_correctly() {
+        for i in 1..=8 {
+            let relay_id = RelayId::new(i).unwrap();
+            let modbus_addr = ModbusAddress::from(relay_id);
+            assert_eq!(modbus_addr.as_u16(), u16::from(i - 1));
+        }
+    }
+}

backend/src/domain/relay/types/relaylabel.rs

@@ -1,3 +1,5 @@
+use std::fmt::Display;
+
 use thiserror::Error;
 
 /// Human-readable label for a relay.
@@ -45,3 +47,60 @@ impl Default for RelayLabel {
         Self(String::from("Unlabeled"))
     }
 }
+
+impl std::fmt::Display for RelayLabel {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_relay_label_new_valid_string() {
+        // Test: RelayLabel::new("Pump") → Ok
+        let result = RelayLabel::new("Pump".to_string());
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap().as_str(), "Pump");
+    }
+
+    #[test]
+    fn test_relay_label_new_valid_max_length() {
+        // Test: RelayLabel::new("A".repeat(50)) → Ok
+        let label_str = "A".repeat(50);
+        let result = RelayLabel::new(label_str.clone());
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap().as_str(), &label_str);
+    }
+
+    #[test]
+    fn test_relay_label_new_empty_fails() {
+        // Test: RelayLabel::new("") → Err(EmptyLabel)
+        let result = RelayLabel::new(String::new());
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), RelayLabelError::Empty));
+    }
+
+    #[test]
+    fn test_relay_label_new_too_long_fails() {
+        // Test: RelayLabel::new("A".repeat(51)) → Err(LabelTooLong)
+        let label_str = "A".repeat(51);
+        let result = RelayLabel::new(label_str);
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), RelayLabelError::TooLong(_)));
+    }
+
+    #[test]
+    fn test_relay_label_default() {
+        let label = RelayLabel::default();
+        assert_eq!(label.as_str(), "Unlabeled");
+    }
+
+    #[test]
+    fn test_relay_label_display() {
+        let label = RelayLabel::new("Test Label".to_string()).unwrap();
+        assert_eq!(format!("{label}"), "Test Label");
+    }
+}