peripherals & power: GPS fixes, poweroff/pwrmgt, upstream SensorManager telemetry

- gps: servers honour gps_enabled at boot (was lost on reboot; zc.gps_set
  upgrades older prefs to on once); sleep lock only while acquiring;
  gps off works on boards without a power line (UART sleep); UI toggles
  and fixes handled on the main thread; configure only after a good init;
  observer parks its GPS; one interval clamp (clampGpsInterval); one
  nav-mode knob (room servers on Air530Z now stationary); GNSS UARTE from DT
- gps: split into ZephyrGPSManager / gps_power / gps_module_cfg
  (+gps_internal.h); dead driver branches, gps_get_diag, sats_* removed
- clock: ZephyrRTCClock writes the hardware RTC itself (clkreboot now
  resets it); MESH_EVENT_RTC_SAVE gone
- power: one power-off path (zephcore_power_off, ZephyrBoard::powerOff),
  poweroff/shutdown CLI, get pwrmgt.*, crash record kept across reboot,
  battery reading cached 10 s, fuel gauge on the current API
- telemetry: upstream SensorManager + CayenneLPP (rounded to nearest),
  one builder for all roles; per-sensor channels, MCU temp always,
  guest gating and inverse mask on servers; sensor list/get/set work;
  BME280/BMP280 also probed at 0x77
- CLI: gps in upstream's form, get gps detail, gps sync

zephcore/helpers/ui/ui_task.h

@@ -216,17 +216,6 @@ void ui_set_battery_provider(uint16_t (*provider)(void));
  */
 void ui_refresh_battery(void);
 
-/**
- * Prepare the device for sys_poweroff(): stop heartbeat LED, blank the
- * display, power off GPS + sensor regulators, hold LoRa in HW reset,
- * configure SENSE on sw0 (nRF only) for button wakeup.
- *
- * Caller is responsible for any shutdown chime BEFORE this call and the
- * final sys_poweroff() AFTER. Both UI variants share this so the System
- * OFF state is consistent regardless of which UI design is compiled in.
- */
-void ui_prepare_for_system_off(void);
-
 /**
  * Register a power-source provider used by ui_auto_shutdown_check().
  * provider() must return true when the device is externally powered
@@ -271,7 +260,7 @@ void ui_set_shutdown_hook(ui_shutdown_fn fn);
  * CONFIG_ZEPHCORE_AUTO_SHUTDOWN_MILLIVOLTS is 0 this is a no-op. Otherwise,
  * if the battery is below the threshold AND not externally powered, it shows
  * a brief warning (3 s on OLED, instant-persist on e-paper) and powers off
- * via ui_prepare_for_system_off() + sys_poweroff().
+ * via zephcore_power_off().
  */
 void ui_auto_shutdown_check(void);