- video: honor the node's fit when the parent reserved a fixed box
  (aspectRatio/sizedBox w+h/stack -> tight constraints). The unconditional
  natural-aspect AspectRatio wrapper made contain/cover/fill render
  identically, so fit buttons looked dead. Loose parents keep the natural
  shape.
- video: fullscreen button in the transport controls (fullscreenButton:
  false to hide) pushes an in-app fullscreen route on the SAME controller —
  playback continues seamlessly.
- tests: fit mapping through re-renders, loose-parent aspect fallback,
  fullscreen round trip, button opt-out.