store: read nodes via index instead of HashMap

- Add get_node() and contains_key() methods that read via redb index
- Migrate all store/ reads to use index lookup
- Remove HashMap cache updates from mutations (write-through to capnp+index only)
- Remove replay_nodes() - load no longer builds HashMap
- Update db_is_healthy to validate by spot-checking offsets
- Fix set_weight bug: now persists weight changes to capnp

Store.nodes HashMap still exists for code outside store/ module,
but store/ itself no longer uses it.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/store/mod.rs

@@ -76,15 +76,37 @@ impl Store {
         graph::build_graph(self)
     }
 
+    /// Get a node by key, reading from capnp via the index.
+    pub fn get_node(&self, key: &str) -> Result<Option<Node>> {
+        let db = self.db.as_ref()
+            .ok_or_else(|| anyhow::anyhow!("store not loaded"))?;
+
+        match index::get_offset(db, key)? {
+            Some(offset) => Ok(Some(capnp::read_node_at_offset(offset)?)),
+            None => Ok(None),
+        }
+    }
+
+    /// Check if a node exists by key.
+    pub fn contains_key(&self, key: &str) -> Result<bool> {
+        let db = self.db.as_ref()
+            .ok_or_else(|| anyhow::anyhow!("store not loaded"))?;
+        index::contains_key(db, key)
+    }
+
     pub fn resolve_key(&self, target: &str) -> Result<String> {
         // Strip .md suffix if present — keys no longer use it
         let bare = strip_md_suffix(target);
 
-        if self.nodes.contains_key(&bare) {
+        if self.contains_key(&bare)? {
             return Ok(bare);
         }
 
-        let matches: Vec<_> = self.nodes.keys()
+        let db = self.db.as_ref()
+            .ok_or_else(|| anyhow::anyhow!("store not loaded"))?;
+        let all_keys = index::all_keys(db)?;
+
+        let matches: Vec<_> = all_keys.iter()
             .filter(|k| k.to_lowercase().contains(&target.to_lowercase()))
             .cloned().collect();