li3: add Lithionics Li3 house battery BMS -> Home Assistant MQTT

Publishes the RV's 12V LiFePO4 house battery to HA via MQTT discovery
(pack voltage, 4 cell voltages, current, SOC, BMS/battery temp, status).
Connects over the battery's BLE HM-10 UART module (service ffe0/char
ffe1, no pairing) using bleak; protocol reverse-engineered from the
com.lithionics.bms Android app's own BLE/parsing code.

Deliberately named li3/ and van-li3-battery, not battery/van-battery —
that name is reserved for the host's own AC/UPS power-supply monitor
(different hardware, unrelated concern).

Self-heals a bluetoothd discovery-state wedge (Discovering stuck "yes",
connects failing with le-connection-abort-by-local) that shows up after
repeated failed connects to this device on the Pi's onboard adapter: the
daemon retries scan/connect internally (MQTT session and HA entities
stay up across retries) and restarts bluetooth.service itself after 3
consecutive scan failures, rate-limited to once per 5 min.

li3/config.json.example

@@ -0,0 +1,12 @@
+{
+  "ble_address": "B0:D2:78:5C:16:87",
+  "broker": "localhost",
+  "port": 1883,
+  "username": "CHANGE_ME",
+  "password": "CHANGE_ME",
+  "client_id": "van-li3-battery",
+  "device_id": "li3_battery",
+  "device_name": "Li3 Battery",
+  "discovery_prefix": "homeassistant",
+  "publish_interval_s": 15
+}