store: move all capnp code to capnp.rs

Consolidate capnp serialization in one place:
- capnp_enum! and capnp_message! macros
- read_text/read_uuid helpers
- Type-to-capnp mappings
- from_capnp_migrate migration impls

types.rs now only has pure Rust types and helpers.

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

src/hippocampus/store/ops.rs

@@ -2,7 +2,7 @@
 //
 // CRUD (upsert, delete), maintenance (decay, cap_degree), and graph metrics.
 
-use super::{index, types::*};
+use super::{index, types::*, Store};
 
 use anyhow::{anyhow, bail, Result};
 use std::collections::{HashMap, HashSet};