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/view.rs

@@ -1,6 +1,6 @@
 // Read-only access abstraction for the memory store
 
-use super::types::*;
+use super::{capnp, index, types::*};
 use super::Store;
 
 // ---------------------------------------------------------------------------
@@ -19,21 +19,42 @@ pub trait StoreView {
 
     /// Node weight by key, or the default weight if missing.
     fn node_weight(&self, key: &str) -> f64;
-
-    /// Node content by key.
-    fn node_content(&self, key: &str) -> Option<&str>;
 }
 
 impl StoreView for Store {
     fn for_each_node<F: FnMut(&str, &str, f32)>(&self, mut f: F) {
-        for (key, node) in &self.nodes {
-            f(key, &node.content, node.weight);
+        let db = match self.db.as_ref() {
+            Some(db) => db,
+            None => return,
+        };
+        let keys = match index::all_keys(db) {
+            Ok(keys) => keys,
+            Err(_) => return,
+        };
+        for key in keys {
+            if let Ok(Some(offset)) = index::get_offset(db, &key) {
+                if let Ok(node) = capnp::read_node_at_offset(offset) {
+                    f(&key, &node.content, node.weight);
+                }
+            }
         }
     }
 
     fn for_each_node_meta<F: FnMut(&str, NodeType, i64)>(&self, mut f: F) {
-        for (key, node) in &self.nodes {
-            f(key, node.node_type, node.timestamp);
+        let db = match self.db.as_ref() {
+            Some(db) => db,
+            None => return,
+        };
+        let keys = match index::all_keys(db) {
+            Ok(keys) => keys,
+            Err(_) => return,
+        };
+        for key in keys {
+            if let Ok(Some(offset)) = index::get_offset(db, &key) {
+                if let Ok(node) = capnp::read_node_at_offset(offset) {
+                    f(&key, node.node_type, node.timestamp);
+                }
+            }
         }
     }
 
@@ -46,10 +67,10 @@ impl StoreView for Store {
 
     fn node_weight(&self, key: &str) -> f64 {
         let cfg = crate::config::get();
-        self.nodes.get(key).map(|n| n.weight as f64).unwrap_or(cfg.default_node_weight)
-    }
-
-    fn node_content(&self, key: &str) -> Option<&str> {
-        self.nodes.get(key).map(|n| n.content.as_str())
+        self.get_node(key)
+            .ok()
+            .flatten()
+            .map(|n| n.weight as f64)
+            .unwrap_or(cfg.default_node_weight)
     }
 }