Animate a sprite

Add sprite animation. My sprite sheet: <dimensions, frames per row,
frame size>.

- An animation is a named list of frame indices plus a duration per
  frame, driven by elapsed time — never by frame count.
- Support looping and one-shot animations, with a callback when a
  one-shot finishes.
- Switching animation resets to frame zero, unless I ask it not to.
- Horizontal flip for facing direction, without a second sheet.

Define the animations as data at the top so I can add one without
touching the drawing code.

Then show me how I'd drive it from state: idle when not moving, run
when moving, jump when airborne.