store: redb indexes offsets into capnp log, not full nodes

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>

src/hippocampus/store/mod.rs

@@ -7,17 +7,17 @@
 // redb provides indexed access; Store struct holds in-memory state.
 //
 // Module layout:
-//   types.rs   — Node, Relation, enums, capnp macros, path helpers
-//   view.rs    — StoreView trait for read-only access
-//   persist.rs — load, replay, append (capnp IO)
-//   ops.rs     — mutations (upsert, delete, decay, cap_degree, etc.)
-//   mod.rs     — re-exports, key resolution, ingestion, rendering
+//   types.rs — Node, Relation, enums, capnp macros, path helpers
+//   index.rs — redb index operations
+//   capnp.rs — capnp log IO (load, replay, append, fsck)
+//   ops.rs   — mutations (upsert, delete, rename, etc.)
+//   view.rs  — StoreView trait for read-only access
 
 mod types;
-mod view;
-mod persist;
+mod index;
+mod capnp;
 mod ops;
-pub mod db;
+mod view;
 
 // Re-export everything callers need
 pub use types::{
@@ -27,7 +27,7 @@ pub use types::{
     new_node, new_relation,
 };
 pub use view::StoreView;
-pub use persist::fsck;
+pub use capnp::fsck;
 pub use ops::current_provenance;
 
 use crate::graph::{self, Graph};