Implement teardown contract for ESPWorker

[zekageri]

Summary

• add public isInitialized() const to ESPWorker
• harden teardown state handling by making _initialized atomic and centralizing deinit state reset
• add lifecycle teardown tests (pre-init deinit, idempotent deinit, re-init, destructor teardown)
• add host test stubs for FreeRTOS/heap APIs and wire tests into CMake
• update README and examples to show explicit deinit() usage

Verification

• g++ -std=gnu++17 test/esp_worker_lifecycle_tests.cpp test/worker_test_stubs.cpp src/esp_worker/worker.cpp -Isrc -Itest/stubs -Itest -pthread -o /tmp/esp_worker_lifecycle_tests && /tmp/esp_worker_lifecycle_tests