store: add weight to index, index-only key matching

- KEY_TO_UUID now stores weight (30 bytes: uuid+type+ts+deleted+weight)
- UUID_OFFSETS changed to composite key for O(log n) max-offset lookup
- Add NODES_BY_TYPE index for efficient type+date range queries
- Add for_each_key_weight() to StoreView for index-only iteration
- match_seeds uses index-only path when content not needed
- Fix transaction consistency in ops (single txn for related updates)
- rebuild() now records all uuid→offset mappings for version history
- Backwards compatible: old index formats decoded with default weight

Co-Authored-By: Proof of Concept <poc@bcachefs.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

src/hippocampus/query/engine.rs

@@ -633,7 +633,8 @@ pub fn match_seeds_opts(
     // Build component index: word → vec of (original key, weight)
     let mut component_map: HashMap<String, Vec<(String, f64)>> = HashMap::new();
 
-    store.for_each_node(|key, _content, weight| {
+    // Index-only pass: no capnp reads needed for key matching
+    store.for_each_key_weight(|key, weight| {
         let lkey = key.to_lowercase();
         key_map.insert(lkey.clone(), (key.to_owned(), weight as f64));