Wires the pieces from Commits 1-9 into a running system:
* triedb/pathdb.New: install the bintrieFlatCodec when isVerkle is set,
backed by the same verkle-namespaced db used for trie nodes.
* triedb/pathdb.database.go: drop isVerkle from the noBuild guard so the
bintrie generator (Commit 9) runs on startup, and remove it from the
generateSnapshot call path for the same reason.
* triedb/pathdb.disklayer.revert: hard-fail on bintrie because the
reorg path would replay merkle-shaped origin records against a
per-stem layout. Tracked in BINTRIE_FLAT_STATE_REORG_GAP.md.
* triedb/pathdb.journal: add IsBintrie to journalGenerator (rlp:"optional"
so v3 journals still decode) and make journalProgress a method on
generator so it stamps the active scheme; loadGenerator discards any
journal whose scheme does not match the database, forcing a fresh
regeneration.
* triedb/pathdb.reader: export RawStateReader, a small extension of
database.StateReader that exposes AccountRLP so callers outside the
package can reach the raw flat-state bytes without going through the
slim-RLP decode path that assumes merkle shape.
* core/state.reader: add bintrieFlatReader, the bintrie equivalent of
flatReader. It derives the EIP-7864 stem keys from (addr, slot),
performs two AccountRLP lookups per Account call (BasicData +
CodeHash), and decodes via bintrie.UnpackBasicData. Storage reads go
through a single AccountRLP lookup at the slot's full bintrie key.
* core/state.database.StateReader: dispatch to bintrieFlatReader when
the path database is in verkle mode; merkle path unchanged.
Depends on the lookup sentinel fix in the previous commit; without it
missing-account reads on bintrie misreport as "layer stale".
