eth/protocols/snap: remove unnecessary Sync() loop, drop errPivotStale and resetDownloadState
eth/protocols/snap: move pivot-reorg detection into Sync(), rename checkDeepReorg to isPivotReorged
eth/protocols/snap: don't apply BALs to accounts that haven't been downloaded yet.
eth/protocols/snap: drop empty accounts and zero-value storage on BAL apply
eth/protocols/snap: wipe flat state on sync reset, consolidate reorg detection
eth/protocols/snap: persist Complete=true on sync completion to skip redundant resyncs
eth/protocols/snap: persist catchUp progress incrementally to enable resume
eth/protocols/snap: verify BALs during fetch to route around bad peers, make catchUp cancelable
core/rawdb,eth/protocols/snap: add DeleteSnapshotSyncStatus helper
eth/downloader, eth/protocols/snap: remove healing and genTrie, restructure sync loop for snap/2
eth/protocols/snap: Implement BAL fetching
eth/protocols/snap: create functions for bal verification and apply
eth/downloader,eth/protocols/snap: implement catch-up on pivot
eth/protocols/snap: add tests and fix peer registration for access lists
eth/protocols/snap: add pivot movement integration tests
core, core/state/snapshot: skip snapshot generation after sync completion
eth/protocols/snap: skip new empty accounts in applyAccessList and test
