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/hippocampus/store/index.rs

@@ -99,6 +99,19 @@ pub fn get_uuid_for_key(db: &Database, key: &str) -> Result<Option<[u8; 16]>> {
     }
 }
 
+/// Get all offsets for a UUID (all versions). Returns newest first.
+pub fn get_offsets_for_uuid(db: &Database, uuid: &[u8; 16]) -> Result<Vec<u64>> {
+    let txn = db.begin_read()?;
+    let table = txn.open_multimap_table(UUID_OFFSETS)?;
+    let mut offsets = Vec::new();
+    for entry in table.get(uuid.as_slice())? {
+        offsets.push(entry?.value());
+    }
+    // Sort descending so newest (highest offset) is first
+    offsets.sort_by(|a, b| b.cmp(a));
+    Ok(offsets)
+}
+
 /// Remove a node from the index (key mappings only; UUID history preserved).
 pub fn remove_node(db: &Database, key: &str, _uuid: &[u8; 16]) -> Result<()> {
     let txn = db.begin_write()?;