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>

platformio.ini

@@ -12,9 +12,13 @@
 default_envs = esp32devdbg
 src_dir = src
 
-[env]
+; Shared by every firmware env (they extend it).  Kept out of [env] so the
+; native test env doesn't inherit the Arduino framework and ESP32 libraries.
+[esp32]
 platform =  espressif32 @ 6.13.0
 framework = arduino
+; Unit tests only run on the PC: pio test -e native
+test_ignore = *
 lib_deps = 
 	bblanchon/ArduinoJson@^7.2.2
 	lsatan/SmartRC-CC1101-Driver-Lib@^2.5.7
@@ -43,35 +47,40 @@ monitor_filters =
 ; task on core 1 (with loop()) never competes with network handling.
 [dualcore]
 build_flags =
-	${env.build_flags}
+	${esp32.build_flags}
 	-DCONFIG_ASYNC_TCP_RUNNING_CORE=0
 
 [env:esp32dev]
+extends = esp32
 board = esp32dev
 board_build.partitions = esp32_3MB.csv
 build_flags = ${dualcore.build_flags}
 
 [env:esp32devdbg]
+extends = esp32
 board = esp32dev
 build_type = debug
 board_build.partitions = esp32_3MB.csv
 build_flags = ${dualcore.build_flags}
 
 [env:esp32c3]
+extends = esp32
 board = esp32-c3-devkitm-1
 board_build.partitions = esp32_3MB.csv
 
 [env:esp32s3]
+extends = esp32
 board = esp32-s3-devkitc-1
 build_flags = ${dualcore.build_flags}
 
 [env:esp32c6]
+extends = esp32
 platform = https://github.com/pioarduino/platform-espressif32/releases/download/stable/platform-espressif32.zip
 board = seeed_xiao_esp32c6
 build_type = debug
 board_build.partitions = esp32_huge_app.csv
-build_flags = 
-	${env.build_flags}
+build_flags =
+	${esp32.build_flags}
 	-DCONFIG_FREERTOS_HZ=1000
 	-include "driver/gpio.h"
 lib_deps = 
@@ -86,3 +95,15 @@ lib_deps =
 	; High-level wrappers/servers
 	ESP32Async/ESPAsyncWebServer
 
+; Host-side unit tests: pio test -e native  (needs gcc/g++ on PATH).
+; Only the sources the tests cover are built; test/stubs stands in for Arduino.h.
+[env:native]
+platform = native
+test_framework = unity
+test_build_src = yes
+build_src_filter = -<*> +<Utils.cpp>
+build_flags =
+	-std=gnu++17
+	-Isrc
+	-Itest/stubs
+	-D_POSIX_THREAD_SAFE_FUNCTIONS