Implement teardown contract for ESPEventBus

[zekageri]

Summary

• add public isInitialized() to ESPEventBus
• harden teardown lifecycle by resetting subscriptions via allocator-safe reconstruction (supports usePSRAMBuffers toggles across re-init)
• keep destructor teardown through deinit() and ensure teardown remains idempotent/pre-init safe
• add lifecycle tests for pre-init deinit(), repeated deinit(), and init -> deinit -> init with subscription reset verification
• document explicit deinit() usage in README and example

Validation

• pio test -e esp32dev -f test_eventbus --without-uploading (failed locally: pio command not found in environment)