test(domain): write failing tests for RelayId newtype validation

Tests cover validation requirements for the RelayId newtype:
- Valid relay IDs (1-8 for 8-channel controller)
- Invalid IDs outside valid range
- Smart constructor error handling
- Type-safe ID representation

TDD red phase: Tests fail until RelayId is implemented.

Ref: T017 (specs/001-modbus-relay-control/tasks.md)

backend/src/domain/relay/types.rs

@@ -12,3 +12,45 @@ impl std::fmt::Display for RelayId {
         self.0.fmt(f)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_relay_id_new_valid_lower_bound() {
+        // Test: RelayId::new(1) → Ok(RelayId(1))
+        let result = RelayId::new(1);
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap(), RelayId(1));
+    }
+
+    #[test]
+    fn test_relay_id_new_valid_upper_bound() {
+        // Test: RelayId::new(8) → Ok(RelayId(8))
+        let result = RelayId::new(8);
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap(), RelayId(8));
+    }
+
+    #[test]
+    fn test_relay_id_new_invalid_zero() {
+        // Test: RelayId::new(0) → Err(InvalidRelayId)
+        let result = RelayId::new(0);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_relay_id_new_invalid_nine() {
+        // Test: RelayId::new(9) → Err(InvalidRelayId)
+        let result = RelayId::new(9);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_relay_id_as_u8_returns_inner_value() {
+        // Test: RelayId::as_u8() returns inner value
+        let relay_id = RelayId(5);
+        assert_eq!(relay_id.as_u8(), 5);
+    }
+}

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

@@ -202,7 +202,7 @@
 
 **⚠️ TDD CRITICAL**: Write failing tests FIRST for every type, then implement
 
-- [ ] **T017** [US1] [TDD] Write tests for RelayId newtype
+- [x] **T017** [US1] [TDD] Write tests for RelayId newtype
   - Test: RelayId::new(1) → Ok(RelayId(1))
   - Test: RelayId::new(8) → Ok(RelayId(8))
   - Test: RelayId::new(0) → Err(InvalidRelayId)