fix(ble): stop BLE from coming back up during the pre-reboot window (#11650)

* fix(ble): stop BLE from coming back up during the pre-reboot window

Saving a reboot-requiring config over BLE (e.g. screen timeout) made the
node disconnect, re-advertise, let the phone reconnect, and then drop it
again at the reset. Two causes:

nRF52: admin messages from the phone run synchronously on Bluefruit's BLE
event task, so the BLE_GAP_EVT_DISCONNECTED caused by shutdown() is only
processed after we return - and that handler restarts advertising because
restartOnDisconnect(true) was never cleared. Stopping advertising first is
a no-op while a connection is live (the SoftDevice isn't advertising), so
the deferred event brought it straight back. Clear the restart flag and
stop advertising before dropping the link, mirroring nRF54L15's ble_enabled
gate. This also closes a main-thread race on the shutdown path where
Advertising.stop() could land between connection teardown and Bluefruit's
auto-restart within the same event dispatch.

PowerFSM (all platforms): darkEnter/onEnter/powerEnter/powerExit/serialExit
unconditionally re-enable BLE, so any state transition inside the reboot
window - a button press while the banner is up, the screen timeout, USB
plug/unplug - turned BLE back on after AdminModule had deliberately torn it
down. Route them through a helper that skips the re-enable while
rebootAtMsec/shutdownAtMsec is armed; every writer of those deadlines is an
imminent restart.

* style: trim rationale comments to house 1-2 line limit

The full mechanism is in the original commit message and PR description.

src/PowerFSM.cpp

@@ -195,6 +195,17 @@ static void lsExit()
     t5BacklightWakeFromSleep();
 }
 
+/// Skip the BLE re-enable while a reboot/shutdown is armed: AdminModule tears BLE down before
+/// scheduling the restart, and a state transition in that window would otherwise bring it back up.
+static void setBluetoothEnableUnlessRestarting()
+{
+    if (rebootAtMsec || shutdownAtMsec) {
+        LOG_POWERFSM("Skip BLE enable, restart pending");
+        return;
+    }
+    setBluetoothEnable(true);
+}
+
 static void nbEnter()
 {
     LOG_POWERFSM("State: nbEnter");
@@ -211,7 +222,7 @@ static void nbEnter()
 static void darkEnter()
 {
     LOG_POWERFSM("State: darkEnter");
-    setBluetoothEnable(true);
+    setBluetoothEnableUnlessRestarting();
     if (screen)
         screen->setOn(false);
     // Screen timeout enters DARK; ensure backlight also turns off.
@@ -235,7 +246,7 @@ static void serialExit()
 {
     LOG_POWERFSM("State: serialExit");
     // Turn bluetooth back on when we leave serial stream API
-    setBluetoothEnable(true);
+    setBluetoothEnableUnlessRestarting();
 }
 
 static void powerEnter()
@@ -248,7 +259,7 @@ static void powerEnter()
     } else {
         if (screen)
             screen->setOn(true);
-        setBluetoothEnable(true);
+        setBluetoothEnableUnlessRestarting();
         // within enter() the function getState() returns the state we came from
     }
 }
@@ -266,7 +277,7 @@ static void powerIdle()
 static void powerExit()
 {
     LOG_POWERFSM("State: powerExit");
-    setBluetoothEnable(true);
+    setBluetoothEnableUnlessRestarting();
 }
 
 static void onEnter()
@@ -274,7 +285,7 @@ static void onEnter()
     LOG_POWERFSM("State: onEnter");
     if (screen)
         screen->setOn(true);
-    setBluetoothEnable(true);
+    setBluetoothEnableUnlessRestarting();
 }
 
 static void onIdle()

src/platform/nrf52/NRF52Bluetooth.cpp

@@ -245,8 +245,12 @@ void NRF52Bluetooth::shutdown()
     // Shutdown bluetooth for minimum power draw
     LOG_INFO("Disable NRF52 bluetooth");
     Bluefruit.Security.setPairPasskeyCallback(NRF52Bluetooth::onUnwantedPairing); // Actively refuse (during factory reset)
-    disconnect();
+
+    // Clear the auto-restart flag before dropping the link: our DISCONNECTED event is only processed
+    // after this callback returns and would re-start advertising. startAdv()/resumeAdvertising() re-set it.
+    Bluefruit.Advertising.restartOnDisconnect(false);
     Bluefruit.Advertising.stop();
+    disconnect();
 }
 void NRF52Bluetooth::startDisabled()
 {