stm32wl(rak3172): implement cpuDeepSleep()/shutdown() via STM32LowPower (#10993)

* stm32wl(rak3172): implement cpuDeepSleep()/shutdown() via STM32LowPower

cpuDeepSleep() was an empty stub, so the SDS deep-sleep PowerFSM state
and low-battery shutdown did nothing but leave the CPU running at full
power. Power::shutdown() also didn't include STM32WL in its arch list,
so an explicit shutdown command just logged a FIXME warning.

Adds STM32LowPower as a lib_dep (rak3172 only, mirroring STM32RTC) and
implements cpuDeepSleep() using it. Standby mode is used for both the
finite-wake (SDS/low battery) and forever (shutdown) paths, via
LowPower.shutdown(), with or without an RTC alarm.

If the LSE-backed hardware RTC never came up (stm32wlRtcAvailable()
false), this is a no-op - safer than sleeping without a confirmed wake
source. LowPower.shutdown() resets the MCU on wake and should never
return; if it somehow does, force a reset via HAL_NVIC_SystemReset()
rather than hanging the device silently forever.

Gated behind the existing HAS_LSE flag, so this is inert on every
variant but rak3172.

Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Andrew Yong <me@ndoo.sg>

* feat(stm32wl): Warn and reset when trying to deepSleep without RTC

This is to prevent leaving the firmware catanonic when firmware has run
its shutdown routine but doesn't actually shutdown.

Signed-off-by: Andrew Yong <me@ndoo.sg>

---------

Signed-off-by: Andrew Yong <me@ndoo.sg>

variants/stm32/rak3172/platformio.ini

@@ -20,5 +20,7 @@ lib_deps =
   ${stm32_base.lib_deps}
   # renovate: datasource=github-tags depName=STM32RTC packageName=stm32duino/STM32RTC
   https://github.com/stm32duino/STM32RTC/archive/refs/tags/1.9.0.zip
+  # renovate: datasource=github-tags depName=STM32LowPower packageName=stm32duino/STM32LowPower
+  https://github.com/stm32duino/STM32LowPower/archive/refs/tags/1.5.0.zip
 
 upload_port = stlink