Enhance state validation tool to report data usage in signed state by subtree/virtual map

It would be nice for the state validation tool to:

1. Measure the size of the signed state by subtree
2. Report the absolute and relative sizes (e.g. subtree X is 5gb, 27% of state)
3. Report the number of entities by subtree

Start by locating the state validation tool and its handling of signed state and the virtual map. Determine how subtrees and entities are represented, then define reporting for each subtree's absolute size, percentage of total state, and entity count. Done means the tool emits all three measurements by subtree.

Written by the indexing model from the issue text.