Release ESPTimer vector storage during deinit

[zekageri]

Summary

• replace vector clear() calls in ESPTimer::deinit() with allocator-preserving swap-to-empty
• ensure timer-owned vector capacity is released immediately on deinit()
• keep allocator policy (usePSRAMBuffers) intact when rebuilding empty vectors

Why

clear() destroys elements but keeps vector capacity allocated, so repeated init/deinit cycles can retain memory that appears leaked until object destruction.

Testing

• not run locally in this environment (no PlatformIO/CMake toolchain available)