Fix the odd-sized video example and improve the Mandelbrot demo

[hhoppe]

• The video example used a 65x65 moving circle. Odd dimensions make write_video fall back from yuv420p to yuv444p, which browsers commonly cannot play, so it is now 64x64.
• The Mandelbrot demo now computes a fractional escape time, giving smooth shading instead of banding.
• The notebook is re-executed, so its outputs reflect the changes. The license cell is removed from the notebook, where it is unsightly; the exported mediapy_examples.py keeps its header. The two files are no longer kept in sync with jupytext.

The Mandelbrot change is identical to the one in #89, so the two merge cleanly in either order.

🤖 Generated with Claude Code

[hhoppe]

Unit tests pass on the fork's CI for head commit 9bb4fe4 (Python 3.10 and 3.x): https://github.com/hhoppe/mediapy/actions/runs/36083142114