vindex: incremental verification

Apologies if I misread the documentation and you already have thought about or are planning this:

It would be neat if verifying updates to the summary map could be done incrementally without storing the whole map. Then a lightweight witness could itself sign off on the summary map, without needing to re-compute or store the map itself.

The crypto machinery for this is mostly straightforward. An inclusion proof for a merkle trie (or other authenticated data structure) is almost the same as a modification proof, since you can encode a lookup proof to a leaf in the old trie, and then re-use that to compute the hash of the trie with a different leaf. This update proof can be stored and published the same way as the log.

Happy to prototype if y’all are interested.

Start by reviewing the vindex documentation and its existing summary-map verification flow; the issue names no source files or tests. Establish how proofs are represented and published, then define completion as incremental verification that lets a lightweight witness validate updates without storing or recomputing the whole map.