Implement teardown contract for ESPDate

[zekageri]

Summary

• add isInitialized() to ESPDate with minimal runtime overhead
• harden deinit() to reset ESPDate-owned runtime state and release config buffers
• add lifecycle tests for pre-init deinit(), idempotent deinit(), and init -> deinit -> init
• update README and example usage to show explicit teardown via deinit()

Validation

• git diff --check
• pio test -d esp-date (fails in this environment: pio command not found)