Add MOS gate control via select entity

Adds a Select entity with four options (Normal / Charge Disabled /
Discharge Disabled / Both Disabled) that sends the 0xE1 write command
to the BMS and immediately refreshes sensor state.  Current option is
derived from the mos_charge_enabled / mos_discharge_enabled bits
already parsed on every poll.

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

custom_components/xiaoxiang_bms/const.py

@@ -23,6 +23,12 @@ CMD_VERSION = bytes([0xDD, 0xA5, 0x05, 0x00, 0xFF, 0xFB, 0x77])  # hardware vers
 FRAME_START = 0xDD
 FRAME_END   = 0x77
 
+# MOS control values (XX byte in write command DD 5A E1 02 00 XX CHK_H CHK_L 77)
+MOS_NORMAL        = 0x00  # release software lock — both gates open
+MOS_CHARGE_OFF    = 0x01  # disable charge MOS, keep discharge MOS on
+MOS_DISCHARGE_OFF = 0x02  # disable discharge MOS, keep charge MOS on
+MOS_BOTH_OFF      = 0x03  # disable both charge and discharge MOS
+
 # Response command IDs (byte 1 of frame)
 CMD_ID_GENERAL = 0x03
 CMD_ID_CELL    = 0x04