Initial commit: Xiaoxiang Smart BMS Home Assistant integration

- BLE GATT communication via bleak (UART-over-GATT, service 0xff00)
- Parses 0x03 (general info) and 0x04 (cell voltages) frames
- Protocol verified against official RS485/UART spec V4
- DataUpdateCoordinator with 5s poll interval (configurable 2-60s)
- Auto-reconnect on BLE disconnect
- Config flow: BT auto-discovery + manual MAC entry + options flow
- Sensors: voltage, current, power, SoC, capacity, cycles, temps, cells, cell delta
- HACS-ready (hacs.json, manifest.json)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

custom_components/xiaoxiang_bms/const.py

@@ -0,0 +1,27 @@
+"""Constants for the Xiaoxiang Smart BMS integration."""
+
+DOMAIN = "xiaoxiang_bms"
+
+CONF_ADDRESS = "address"
+CONF_POLL_INTERVAL = "poll_interval"
+
+DEFAULT_POLL_INTERVAL = 5   # seconds — fast for solar/energy monitoring
+MIN_POLL_INTERVAL = 2
+MAX_POLL_INTERVAL = 60
+
+# GATT UUIDs (Xiaoxiang BMS UART-over-GATT)
+UART_SERVICE_UUID = "0000ff00-0000-1000-8000-00805f9b34fb"
+RX_CHAR_UUID      = "0000ff01-0000-1000-8000-00805f9b34fb"  # BMS → HA (notify)
+TX_CHAR_UUID      = "0000ff02-0000-1000-8000-00805f9b34fb"  # HA → BMS (write)
+
+# Request frames: [0xDD, 0xA5, CMD, 0x00, CHK_HI, CHK_LO, 0x77]
+CMD_GENERAL = bytes([0xDD, 0xA5, 0x03, 0x00, 0xFF, 0xFD, 0x77])  # pack info
+CMD_CELL    = bytes([0xDD, 0xA5, 0x04, 0x00, 0xFF, 0xFC, 0x77])  # cell voltages
+
+# Frame markers
+FRAME_START = 0xDD
+FRAME_END   = 0x77
+
+# Response command IDs (byte 1 of frame)
+CMD_ID_GENERAL = 0x03
+CMD_ID_CELL    = 0x04