merge: bring the record-lookup work onto the TxIndex reader/writer refactor

Both conflicts were additive, not semantic: the crate root export list gained
entries on each side, and the block-source file gained a BlockTreeAdapter impl on
main where this branch had deleted the duplicate lookup helpers. The merge
resurrected those helpers alongside the import that replaced them, so they are
deleted again here.

The premise still holds after the refactor: record_for_hash on the new main still
walks the log linearly in all four places this branch replaces.

Re-ran the mutation audit rather than trusting the tests for staying green
through a merge that moved the code they pin. Both mutations still turn the
intended tests red. One change is worth noting: under the hash-ignoring mutation
node used to fail a test too, and no longer does -- main routing that path through
BlockTreeAdapter cost node its detector. The rpc side still catches it twice.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: rabbitson87