Implement teardown contract for ESPStore

[zekageri]

Summary

• add ESPStore::deinit() and ESPStore::isInitialized() const
• make destructor delegate teardown and remove ready() (no compatibility alias)
• make teardown idempotent, pre-init safe, and release runtime/default state
• add lifecycle tests for pre-init deinit, double deinit, and init->deinit->init
• update README/examples/changelog to show explicit deinit usage

Validation

• unable to run on-device tests in this environment (arduino-cli/pio unavailable)