Restructure store module with clearer file names:
- persist.rs → capnp.rs (capnp log IO)
- db.rs → index.rs (redb index operations)
redb now stores key → offset mapping, not serialized nodes.
Mutations record the offset after appending to capnp log.
rebuild_index scans capnp log to reconstruct the index.
The HashMap still exists for now; next step is to use the
index for lookups and remove it.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
