feat: theme factor fields state their own range for the editor #192

What

The theme editor's sliders cap every factor field at 1. The motion fields get a hand-coded range of 0 to 2, and the overflow choice from #177 only works because its two choices are 0 and 1. A factor field cannot say its own range, so a future choice with more than two options, or any factor that may exceed 1, gets an unusable slider.

Acceptance

• A factor field in the theme's field table states its own range, and the editor's slider uses it
• The motion fields' hand-coded range is replaced by the table's
• A test covers a field with a range other than 0 to 1