- validateTableName: guard against empty string input to prevent malformed SQL
- Save/PruneVersions/Delete: use mu.Lock to serialize SELECT MAX(version)+INSERT
preventing duplicate version numbers under concurrent goroutines
- Load/LoadVersion/ListVersions: retain mu.RLock to allow concurrent reads
- All methods: add nil db check after acquiring lock to fail fast after Close()
- NewSQLiteCheckpointer: set SetMaxOpenConns(1)+SetMaxIdleConns(1) to ensure
PRAGMA WAL/busy_timeout takes effect on the single shared connection and
eliminate SQLITE_BUSY errors under concurrent writes
- ListVersions: initialize result as make([]CheckpointVersion, 0) instead of
nil slice for consistent caller semantics
- PruneVersions: add comment explaining the double threadID parameter
unclesam