store: convert more callers to use RELS index

Convert remaining Vec users to index-based access:
- memory.rs: MemoryNode::from_store uses Store::neighbors()
- graph.rs: orphan detection uses for_each_relation
- local.rs: normalize_strengths uses for_each_relation + set_link_strength

Add Store::neighbors() method and index::get_offsets_for_uuid().

Cleanup:
- for_each_relation: build both uuid↔key maps in one pass
- cap_degree: consolidate key/uuid/degree collection

Remaining Vec uses: admin.rs (fsck, dedup), capnp.rs (load path).

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/subconscious/digest.rs

@@ -176,12 +176,6 @@ pub fn apply_digest_links(store: &mut Store, links: &[DigestLink]) -> (usize, us
 
         if source == target { skipped += 1; continue; }
 
-        // Check if link already exists
-        let exists = store.relations.iter().any(|r|
-            r.source_key == source && r.target_key == target && !r.deleted
-        );
-        if exists { skipped += 1; continue; }
-
         let source_uuid = match store.get_node(&source).ok().flatten() {
             Some(n) => n.uuid,
             None => { skipped += 1; continue; }
@@ -191,6 +185,12 @@ pub fn apply_digest_links(store: &mut Store, links: &[DigestLink]) -> (usize, us
             None => { skipped += 1; continue; }
         };
 
+        // Check if link already exists via index
+        let exists = store.neighbors(&source).ok()
+            .map(|n| n.iter().any(|(k, _)| k == &target))
+            .unwrap_or(false);
+        if exists { skipped += 1; continue; }
+
         let rel = new_relation(
             source_uuid, target_uuid,
             store::RelationType::Link,