Drains the binaryHasher's LeafProducer side-channel in StateDB.commit and
threads the stem writes through stateUpdate.encodeBinary into the pathdb
state set as per-offset accountData entries (key = stem||offset, value =
32-byte leaf or nil for clears).
The flat-state codec gains a Flush method that owns the in-memory→disk
write path, replacing the codec-agnostic per-entry loop in writeStates.
The merkle codec preserves its historical per-entry behavior verbatim;
the bintrie codec aggregates per-offset writes by stem so each stem hits
disk via a single read-modify-write, satisfying the codec's pre-aggregation
requirement and updating the clean cache with the merged blob it just
produced (no extra disk read).
stateUpdate.encodeBinary returns empty origin maps for the bintrie path:
state-history rollback for bintrie is deferred to a follow-up PR (see
BINTRIE_FLAT_STATE_REORG_GAP.md), and the diskLayer.revert path will
panic before consuming origins anyway.
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