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/van-li3-battery.service

@@ -0,0 +1,17 @@
+# /etc/systemd/system/van-li3-battery.service
+[Unit]
+Description=Publish Li3 BMS battery telemetry to Home Assistant via MQTT
+After=network-online.target bluetooth.target mosquitto.service
+Wants=network-online.target
+
+[Service]
+ExecStart=/usr/local/sbin/van-li3-battery
+# The script retries BLE scan/connect internally (including self-healing a
+# wedged bluetoothd discovery state — see the script's docstring) and only
+# exits on MQTT-level failure, which shouldn't happen in normal operation.
+# Restart=always is a backstop for that case, matching van-gps-owntracks.
+Restart=always
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target