test: add native Unity env with Utils tests

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>

test/stubs/Arduino.h

@@ -0,0 +1,20 @@
+// Minimal stand-in for the Arduino core so firmware sources can be built for
+// the native test env (pio test -e native).  Only add what tested code uses.
+#ifndef arduino_stub_h
+#define arduino_stub_h
+#include <cctype>
+#include <cmath>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <ctime>
+
+// Fixed so anything formatted from it is deterministic.
+inline unsigned long millis() { return 0; }
+inline bool getLocalTime(struct tm *info, uint32_t ms = 5000) {
+  (void)ms;
+  time_t now = time(nullptr);
+  return localtime_r(&now, info) != nullptr;
+}
+#endif