feat(application): HealthMonitor service and hardware integration test

Add HealthMonitor service for tracking system health status with
comprehensive state transition logic and thread-safe operations.
Includes 16 unit tests covering all functionality including concurrent
access scenarios.

Add optional Modbus hardware integration tests with 7 test cases for
real device testing. Tests are marked as ignored and can be run with

Ref: T034, T039, T040 (specs/001-modbus-relay-control/tasks.org)

justfile

@@ -31,7 +31,10 @@ release-run:
 	cargo run --release
 
 test:
-	cargo test
+	cargo test --all --all-targets
+
+test-hardware:
+	cargo test --all --all-targets -- --ignored
 
 coverage:
 	mkdir -p coverage