Adds an [env:native] for host-side unit tests (pio test -e native) with
a minimal Arduino.h stub, and covers the Utils helpers: _trim,
toBoolean, SETCHARPROP, sortArray and the Timestamp conversions.
The firmware envs now extend a shared [esp32] section instead of [env]
so the native env doesn't inherit the Arduino framework, the ESP32
libraries or the build scripts.
The half-hour calcTZOffset cases fail: +05:30 comes back as 2300.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
