Restructure the FsStore on-disk layout so that all `<HASH>.bin` node
files live under a `nodes/` subdirectory of the store root, instead of
being interleaved with metadata directories (`_index/`, `_store.db`,
`_meta`) at the top level.
Pre-existing flat-layout stores are auto-migrated on first open: any
`.bin` files found in the store root are renamed (not copied) into
`nodes/`. Migration is idempotent and safe to re-run.
Fixes#84
xiaoju