Implemented the Relay aggregate as the primary domain entity for relay
control operations. Added RelayLabel newtype for validated human-readable
relay labels.
Relay aggregate features:
- Construction with id, state, and optional label
- State control methods: toggle(), turn_on(), turn_off()
- Accessor methods: id(), state(), label()
- All methods use const where possible for compile-time optimization
RelayLabel newtype features:
- Validation: non-empty, max 50 characters
- Smart constructor with Result-based error handling
- Default implementation: "Unlabeled"
- Transparent representation for zero-cost abstraction
Additional changes:
- Made RelayId derive Copy for ergonomic value semantics
- All public APIs include documentation and #[must_use] attributes
TDD phase: GREEN - Tests pass for Relay aggregate (T021 tests now pass)
Ref: T022, T024 (specs/001-modbus-relay-control/tasks.md)
Created test suite for Relay entity covering construction, state toggling,
and explicit state control methods. Tests intentionally fail as Relay
struct is not yet implemented.
Tests cover:
- Relay::new() with id, state, and optional label
- toggle() flipping state between On/Off
- turn_on() setting state to On
- turn_off() setting state to Off
TDD phase: RED - Tests written, implementation pending (T022)
Ref: T021 (specs/001-modbus-relay-control/tasks.md)
