Add optional flag to output node counts per level

[carpalsgrabby]

Summary

For some debugging and visualization workflows, it’s helpful to see the number of nodes at each tree level (from root to leaves). The script currently does not expose this information.

Proposal

• Add a new flag: --levels (boolean)
• When enabled, output a list such as:
  • level 0 (root): 1 node
  • level 1: arity nodes
  • …
  • level h: arity**h nodes (full leaves)
• Include this in both human-readable and JSON modes.

Benefit

Gives users a more complete understanding of the tree structure and helps debugging state commitment models.