Prototype and de-risk Partitioned Binary Tree

Collaborate with the EF on prototyping and de-risking the [Partitioned Binary Tree](https://cperezz.github.io/pbt-spec/) (PBT). Specifically:

- Use our expertise in data models and algorithms to critique the design
- Prototype an implementation in Erigon
- Measure and optimize its performance (contrast with the current Merkle–Patricia Trie)
- Help to de-risk it for other clients – especially the transition

So it's a lot of R&D and project work, not simply coding.

Start with the linked Partitioned Binary Tree specification and compare its design with Erigon’s current Merkle–Patricia Trie; the issue names no source files or tests. Scope the prototype, performance measurements, and transition risks with the EF, then document whether the approach is viable for other clients.

Written by the indexing model from the issue text.