LoraDB v0.6: columnar snapshots and managed WAL checkpoints

LoraDB v0.6 is a persistence hardening release.
v0.3 introduced snapshots. v0.4 introduced WAL recovery. v0.5 made the engine stream results and tightened container-backed persistence. v0.6 brings those persistence pieces into a cleaner shape: columnar snapshots, managed WAL checkpoints, and binding APIs that describe the same operational model from every runtime.
The headline is simple: the persistence story is no longer "one file" and "one log" as separate ideas. Snapshots are now the checkpoint artifact the WAL can lean on.



