split capnp_store.rs into src/store/ module hierarchy

capnp_store.rs (1772 lines) → four focused modules:
  store/types.rs  — types, macros, constants, path helpers
  store/parse.rs  — markdown parsing (MemoryUnit, parse_units)
  store/view.rs   — StoreView trait, MmapView, AnyView
  store/mod.rs    — Store impl methods, re-exports

new_node/new_relation become free functions in types.rs.
All callers updated: capnp_store:: → store::

src/query.rs

@@ -21,7 +21,7 @@
 //   key ~ 'journal.*' AND degree > 10 | count
 //   * | sort weight asc | limit 20
 
-use crate::capnp_store::{NodeType, Provenance, RelationType, Store};
+use crate::store::{NodeType, Provenance, RelationType, Store};
 use crate::graph::Graph;
 use regex::Regex;
 use std::collections::BTreeMap;