* Clean up the fractals directory
Add a directory README plus two new, fully doctested fractal generators.
- README.md: overview table of every fractal, how to run them, and further
reading. Distinguishes the visual (turtle/matplotlib) demos from the
pure-computation ones that run under doctest in CI.
- sierpinski_carpet.py: integer-arithmetic Sierpinski carpet. is_filled(),
generate_carpet() and count_filled_cells() are all doctested; no display
needed.
- barnsley_fern.py: the Barnsley fern iterated function system. transform(),
choose_transformation() and generate_fern() are doctested and made
deterministic via an optional seed; matplotlib plotting is optional and
guarded behind __main__.
* fractals: inline demo code under __main__ (address keeper-bot doctest note)
