Add Sombra Shades window shade sub-driver

[Woofser]

Summary

Adds a sub-driver to the zigbee-window-treatment driver for Sombra Shades
Zigbee window shade motors (manufacturer "Sombra Shades").

Details

• Position is reported and commanded through the standard Window Covering
  cluster (0% = open, 100% = closed); no inversion.
• Opening/Closing status is inferred from the direction of position change,
  so it works across Sombra motor variants (battery and DC) regardless of
  any manufacturer-specific motion cluster. A short settle timer emits the
  resting state once movement stops.
• Battery level via the Power Configuration cluster; power source via the
  Basic cluster.
• Registered through the existing lazy-load mechanism in sub_drivers.lua.

Testing

• Unit tests in test_zigbee_window_treatment_sombra.lua.
• Verified on physical Sombra hardware (battery and DC motors): open, close,
  pause, set level, preset, position reporting, battery, and power source.

[CLAassistant]

All committers have signed the CLA.