Implement teardown contract for ESPTimer

[zekageri]

Summary

• add isInitialized() const to ESPTimer and remove the clearTimer backward-compatibility alias
• keep teardown centered on deinit() and align docs/examples with explicit teardown usage
• add lifecycle coverage for pre-init deinit(), repeated deinit(), and init -> deinit -> init

Verification

• pio test -d /home/zekageri/projects/esptoolkit/libs/esp-timer (fails in this environment: pio not installed)
• cmake -S /home/zekageri/projects/esptoolkit/libs/esp-timer -B /tmp/esp-timer-cmake-check && cmake --build /tmp/esp-timer-cmake-check (fails in this environment: cmake not installed)