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/mod.rs

@@ -7,9 +7,9 @@
 // redb provides indexed access; Store struct holds in-memory state.
 //
 // Module layout:
-//   types.rs — Node, Relation, enums, capnp macros, path helpers
+//   types.rs — Node, Relation, enums, path/time helpers
+//   capnp.rs — serialization macros, log IO (load, replay, append, fsck)
 //   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
 
@@ -23,7 +23,7 @@ mod view;
 pub use types::{
     memory_dir, nodes_path,
     now_epoch, epoch_to_local, format_date, format_datetime, format_datetime_space, compact_timestamp, today,
-    Node, Relation, NodeType, RelationType, Store,
+    Node, Relation, NodeType, RelationType,
     new_node, new_relation,
 };
 pub use view::StoreView;
@@ -32,6 +32,7 @@ pub use ops::current_provenance;
 
 use crate::graph::{self, Graph};
 
+use std::collections::HashMap;
 use anyhow::{bail, Result};
 
 /// Strip .md suffix from a key, handling both bare keys and section keys.
@@ -45,6 +46,31 @@ pub fn strip_md_suffix(key: &str) -> String {
     }
 }
 
+// The full in-memory store
+pub struct Store {
+    pub nodes: HashMap<String, Node>,          // key → latest node
+    pub uuid_to_key: HashMap<[u8; 16], String>, // uuid → key (rebuilt from nodes)
+    pub relations: Vec<Relation>,              // all active relations
+    /// Log sizes at load time — used for staleness detection.
+    pub(crate) loaded_nodes_size: u64,
+    pub(crate) loaded_rels_size: u64,
+    /// redb index database
+    pub(crate) db: Option<redb::Database>,
+}
+
+impl Default for Store {
+    fn default() -> Self {
+        Store {
+            nodes: HashMap::new(),
+            uuid_to_key: HashMap::new(),
+            relations: Vec::new(),
+            loaded_nodes_size: 0,
+            loaded_rels_size: 0,
+            db: None,
+        }
+    }
+}
+
 impl Store {
     pub fn build_graph(&self) -> Graph {
         graph::build_graph(self)