Implement teardown lifecycle contract for ESPCpuMonitor

[zekageri]

Summary

• add isInitialized() to ESPCpuMonitor as a constant-time ownership check
• harden deinit() teardown by clearing runtime-owned callback/history state and resetting smoothing/temperature state
• guard sampling paths (sampleNow and timer callback) with initialized-state checks
• add on-device lifecycle tests for pre-init deinit, repeated deinit, re-init, and destructor cleanup
• document teardown usage and lifecycle guarantees in README
• add PlatformIO test runner config for on-device tests

Notes

• host-side execution of these tests is intentionally disabled because this library depends on ESP32 FreeRTOS idle hooks and esp_timer
• no backward-compatibility aliases were added per request