capnp_store: remove dead code, consolidate CRUD API

Dead code removed:
- rebuild_uuid_index (never called, index built during load)
- node_weight inherent method (all callers use StoreView trait)
- node_community (no callers)
- state_json_path (no callers)
- log_retrieval, log_retrieval_append (no callers; only _static is used)
- memory_dir_pub wrapper (just make memory_dir pub directly)

API consolidation:
- insert_node eliminated — callers use upsert_node (same behavior
  for new nodes, plus handles re-upsert gracefully)

AnyView StoreView dispatch compressed to one line per method
(also removes UFCS workaround that was needed when inherent
node_weight shadowed the trait method).

-69 lines net.

src/digest.rs

@@ -19,7 +19,7 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 
 fn memory_dir() -> PathBuf {
-    capnp_store::memory_dir_pub()
+    capnp_store::memory_dir()
 }
 
 fn episodic_dir() -> PathBuf {