Map explorer: opaque/see-through toggle + transition-speed (kappa) slider

- Add a Sphere opaque/see-through toggle: opaque (default) hides back-hemisphere
  curves, graticule, arc, and occluded markers to avoid confusing overlap;
  see-through restores the dim back-side depth cue.
- Add a "Transition speed (k)" slider (real-model engine): the slow-sweep rate
  along the arc. Lower k = gradual onset/offset ramps; higher k = sharper.
- Fix frequency calibration to be k-independent: measure the ictal cycle period
  at the core with parameters held fixed (carry state onto the limit cycle, then
  dwell) instead of on the sweeping trajectory. Frequency now holds across the
  whole k range for smooth dynamotypes.
- Default opens on a smooth SupH -> FLC seizure (clean showcase for the slider).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: mb-westover