Align atik/stx camera interfaces and add tests

- start(): rename dt→datetime kwarg in atik (matches stx)
- full/half/small_frame(): return [w, h] in both drivers
- all: merged keys — both dicts now contain ambient, setpoint,
  temperature, cooler, fan, binning, max_range, xystart, xyend,
  xrange, yrange, center, state, description
- atik: track _subframe state to compute xystart/xyend/xrange/yrange
- atik: filter.setter added as no-op (no filter wheel)
- tests/test_camera_stx.py, tests/test_camera_atik.py: 30 unit tests

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