Sort out InkHUD applying-changes notifications

notifyApplyingChanges() was doing two jobs at its call sites in MenuApplet.cpp:
signalling a live change that is applied without restarting, and warning of an
imminent reboot. Audited every site. The finding is that none of them are
redundant, so this records why rather than removing anything.

The two live-change calls - LoRa region and modem preset - cover the seconds the
e-ink takes to redraw while the radio reconfigures, and nothing else raises them.
The reboot-path calls warn before the display goes. requestReboot() cannot absorb
those: it deliberately carries no UI, because BaseUI renders its own notice at
draw time from rebootAtMsec, while e-ink only draws when pushed.

Also worth recording: the existing notifyReboot Observable (sleep.h, fired from
Power::reboot) is a different moment - reboot execution, not scheduling - and
InkHUD already observes it to save settings and shut applets down. A new
"reboot scheduled" observable to centralise these calls would be a third reboot
signal for no functional gain, so it is not added.

clod helped too

Author: NomDeTom

src/graphics/niche/InkHUD/Applets/System/Menu/MenuApplet.cpp

@@ -342,7 +342,9 @@ static void applyLoRaRegion(meshtastic_Config_LoRaConfig_RegionCode region)
         snprintf(moduleConfig.mqtt.root, sizeof(moduleConfig.mqtt.root), "%s/%s", default_mqtt_root, myRegion->name);
         changes |= SEGMENT_MODULECONFIG;
     }
-    // Notify UI that changes are being applied
+    // Live change, not a reboot: covers the e-ink redraw while the radio reconfigures. See the
+    // two-jobs note on InkHUD::notifyApplyingChanges() - this call is not interchangeable with the
+    // ones that sit beside CONFIG_APPLY_REBOOT below.
     InkHUD::InkHUD::getInstance()->notifyApplyingChanges();
     // LoRa config applies live via configChanged observer -> RadioInterface::reconfigure(); no reboot needed.
     service->applyConfigChange(changes, CONFIG_APPLY_RADIO);
@@ -372,7 +374,7 @@ static void applyLoRaPreset(meshtastic_Config_LoRaConfig_ModemPreset preset)
     // LoRa config applies live via configChanged observer -> RadioInterface::reconfigure(); no reboot needed.
     service->applyConfigChange(SEGMENT_CONFIG, CONFIG_APPLY_RADIO);
 
-    // Notify UI that changes are being applied
+    // Live change, not a reboot (see applyLoRaRegion above).
     InkHUD::InkHUD::getInstance()->notifyApplyingChanges();
 }
 

src/graphics/niche/InkHUD/InkHUD.h

@@ -49,6 +49,26 @@ class InkHUD
     void setDriver(Drivers::EInk *driver);
     void setDisplayResilience(uint8_t fastPerFull = 5, float stressMultiplier = 2.0);
     void addApplet(const char *name, Applet *a, bool defaultActive = false, bool defaultAutoshow = false, uint8_t onTile = -1);
+
+    /** Show the "applying changes" holding screen.
+     *
+     * Called at the moment a config change is *initiated*, and it serves two distinct jobs at its
+     * call sites in MenuApplet.cpp. Do not treat them alike when refactoring:
+     *
+     *  - a **live** change, applied without restarting (LoRa region and modem preset). The screen
+     *    covers the seconds the e-ink takes to redraw. These calls must stay: nothing else raises
+     *    them.
+     *  - an imminent **reboot**, where it warns before the display goes. These sit next to an
+     *    applyConfigChange(..., CONFIG_APPLY_REBOOT).
+     *
+     * Neither is redundant with anything, which is the point worth recording. MeshService's
+     * requestReboot() deliberately carries no UI: BaseUI renders its own notice at draw time from
+     * rebootAtMsec, whereas e-ink only draws when pushed, so InkHUD has to be told explicitly.
+     * And the existing `notifyReboot` Observable (sleep.h, fired from Power::reboot) is a
+     * *different moment* - reboot execution, not scheduling - which InkHUD already observes to save
+     * settings and shut applets down. Centralising these calls onto a new "reboot scheduled"
+     * observable would add a third reboot signal for no functional gain.
+     */
     void notifyApplyingChanges();
 
     void begin();