Adds generateBinTrieStems, the bintrie analogue of generateAccounts. It
opens the bintrie via a sha256-aware bintrieDiskStore (the merkle disk
store would always fail root validation against a binary node), iterates
all leaves with binaryNodeIterator, aggregates them into per-stem
builders, and emits one stem blob per stem boundary.
Resume support is structural: ctx.marker is fed straight to the trie's
NodeIterator, which uses binaryNodeIterator.seek (Commit 1) to position
on the first leaf >= marker. Range proofs are deliberately skipped — the
bintrie's Prove path is unimplemented and an iteration-only generation
cycle is acceptable for a one-time startup cost.
A bintrieGeneratorContext mirrors generatorContext but is much smaller:
no holdable iterators (we walk the trie, not the existing flat state)
and no two-tier marker (the bintrie key space is unified). checkAndFlushBin
journals progress as a single 32-byte (stem || offset) key so resume
can pick up mid-stem.
generator.run dispatches on codec type so callers see a uniform
lifecycle whether the underlying scheme is merkle or bintrie.
