Add --subtree-height option to compute subtree node counts

## Summary
Users exploring state commitment trees often want to compute properties of a subtree at height `k` (e.g., for partial commitments). The script currently only computes full-tree stats.

## Proposal
- Add a flag: `--subtree-height N`
- Compute:
  - nodes in subtree
  - leaves in subtree
  - internal nodes in subtree

## Benefit
Helps model sharded or incremental commitment structures.
