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.
Introduce flatStateCodec, a small interface that captures the
trie-specific aspects of flat-state storage: key derivation from
(address, slot), persistence of account/storage entries, clean-cache
key disambiguation, iterator setup, and progress-marker handling.
Mirrors the existing nodeHasher pattern and complements the Hasher
interface from state-hasher-iface-2 (which abstracts trie-side hashing
and commit). The codec is stored on Database alongside the existing
hasher field, ready to be threaded through the flat-state call sites
(disklayer, flush, generator, reader) in the next commit.
Provides merkleFlatCodec, a thin wrapper over the existing rawdb
snapshot accessors and helpers. This is a pure refactor: behavior is
unchanged. The bintrie-side codec implementation is added in a later
commit, after all call sites have been routed through the abstraction.
