store: wire up redb updates on mutations

Mutations (upsert_node, upsert_provenance, delete_node, rename_node)
now update redb indices atomically with capnp log appends, under the
same StoreLock.

Also removes dead cmd_import command and the parse.rs module it depended on.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/store/mod.rs

@@ -8,14 +8,12 @@
 //
 // Module layout:
 //   types.rs   — Node, Relation, enums, capnp macros, path helpers
-//   parse.rs   — markdown → MemoryUnit parsing
 //   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
 
 mod types;
-mod parse;
 mod view;
 mod persist;
 mod ops;
@@ -28,7 +26,6 @@ pub use types::{
     Node, Relation, NodeType, RelationType, Store,
     new_node, new_relation,
 };
-pub use parse::{MemoryUnit, parse_units};
 pub use view::StoreView;
 pub use persist::fsck;
 pub use ops::current_provenance;