0.2.2 - fix base-layer effect retention

What's Changed

• Fixed #6: an effect set on the base light entity used to stick around
  forever -- a manual colour change afterward left the stale effect key
  merged into every subsequent light.turn_on call, with no way to clear
  it. Colour updates now implicitly cancel a stored effect unless a new
  effect is given in the same call.
• Added a pytest test suite (coordinator layer-priority logic, light/sensor
  entities, config flow, setup/unload lifecycle) plus a GitHub Actions
  workflow to run it on every push/PR.
• Prepared the repository for HACS: added hacs.json, a brand/icon.png,
  and the hacs/action + hassfest validation workflows required for
  default-store submission.

Full Changelog: 0.2.1...0.2.2