Add --plot option to visualize tree padding or size distribution

## Summary
For large trees, it’s useful to visualize how padding increases with different arities. A `--plot` mode (using matplotlib) would allow users to generate simple graphs.

## Proposal
- Add a `--plot` option that shows:
  - padding fraction vs. height
  OR
  - node counts per level
- Save plot to a file, e.g. `tree_layout.png`.

## Benefit
Improves usability for research and presentations.
