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/search.rs

@@ -4,7 +4,7 @@
 // supports circumscription parameter for blending associative vs
 // causal walks, and benefits from community-aware result grouping.
 
-use crate::capnp_store::StoreView;
+use crate::store::StoreView;
 use crate::graph::Graph;
 
 use std::collections::{HashMap, HashSet, VecDeque};