fix(ble): reliably expose and update BLE battery level (BAS) (#10622)

* fix(ble): reliably expose and update BLE battery level (BAS)

The Battery Service (0x180F / 0x2A19) is now wired up per the Bluetooth
BAS spec: the Battery Level characteristic always holds a valid 0-100
value and is pushed on change.

- NimBLE: seed an initial level at setup and cache the value on every
  update so a READ returns the current level even while disconnected;
  only notify when a client is connected.
- Power: mirror the battery level to the Battery Service from
  readPowerStatus() on change, so it updates independent of GPS/position
  events (previously the only push path was MeshService).

Also fixes two regressions the above would otherwise introduce:

- NimBLE use-after-free: BLEDevice::deinit(true) frees the GATT objects
  but left the global BatteryCharacteristic dangling. Several AdminModule
  paths (e.g. serial-config entry) deinit BLE while config.bluetooth.enabled
  stays true, so the periodic push would deref freed memory. Null the
  pointer in deinit().
- NRF52: guard blebas.write() on the nrf52Bluetooth instance so the new
  periodic push can't call it before the Battery Service is begun in setup().

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(ble): clamp BAS battery level to 0-100 and skip redundant updates

Address review feedback on the Battery Level characteristic (0x2A19):

- Clamp the value to the BAS-mandated 0-100 range at the platform write
  boundary (NimBLE seed + update, NRF52 update), so a misbehaving battery
  backend can't put an out-of-range value on the characteristic.
- Skip the write/notify when the level is unchanged, so repeated callers
  (e.g. MeshService refresh paths) don't emit redundant notifications.
- Simplify Power.cpp to a direct guarded call now that clamping and
  de-duplication live at the boundary, which also removes the implicit
  int->uint8_t narrowing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/Power.cpp

@@ -14,6 +14,7 @@
  * For more information, see: https://meshtastic.org/
  */
 #include "power.h"
+#include "BluetoothCommon.h"
 #include "MessageStore.h"
 #include "NodeDB.h"
 #include "PowerFSM.h"
@@ -962,6 +963,10 @@ void Power::readPowerStatus()
         lastLogTime = millis();
     }
     newStatus.notifyObservers(&powerStatus2);
+
+    // Mirror battery level to the BLE Battery Service (0x2A19); the platform layer clamps and dedupes.
+    if (hasBattery == OptTrue)
+        updateBatteryLevel(powerStatus2.getBatteryChargePercent());
 #ifdef DEBUG_HEAP
     if (lastheap != memGet.getFreeHeap()) {
         // Use stack-allocated buffer to avoid heap allocations in monitoring code