Route the flatStateCodec from Database through every flat-state call
site so that the trie-specific aspects of persistence and key derivation
live behind a single abstraction. Pure refactor: merkle behavior and
on-disk layout are unchanged because the only codec wired up is
merkleFlatCodec, whose methods are thin wrappers over the existing
rawdb accessors.
Threaded sites:
disklayer.account/storage use codec.{Read,AccountCacheKey,
StorageCacheKey} instead of direct
rawdb calls and bare hash slicing.
flush.writeStates takes a codec parameter; persistence
goes through codec.{Write,Delete}
{Account,Storage}.
buffer.flush carries the codec down into writeStates.
states.write/dbsize takes the codec for prefix-size
accounting.
generate.go (g.codec) the generator owns a codec, used by
generateAccounts/generateStorages
callbacks; the unused top-level
splitMarker helper is removed in favor
of codec.SplitMarker.
context.go the generator context owns the codec
and uses codec.{AccountPrefix,
StoragePrefix,Account/StorageKeyLength}
to construct iterators.
reader.go (HistoricalState) uses codec.{Account,Storage}Key for
caller-side key derivation.
The marker comparisons in writeStates remain merkle-shaped (two-tier
account+storage marker) because the bintrie path will use a separate
writer over single-tier stem markers in a later commit.
All existing pathdb tests pass.
In this pull request, snapshot generation in pathdb has been ported from
the legacy state snapshot implementation. Additionally, when running in
path mode, legacy state snapshot data is now managed by the pathdb
based snapshot logic.
Note: Existing snapshot data will be re-generated, regardless of whether
it was previously fully constructed.
