clippy: fix all warnings across all binaries

- &PathBuf → &Path in memory-search.rs signatures
- Redundant field name in graph.rs struct init
- Add truncate(false) to lock file open
- Derive Default for Store instead of manual impl
- slice::from_ref instead of &[x.clone()]
- rsplit_once instead of split().last()
- str::repeat instead of iter::repeat().take().collect()
- is_none_or instead of map_or(true, ...)
- strip_prefix instead of manual slicing

Zero warnings on `cargo clippy`.

src/capnp_store.rs

@@ -49,7 +49,7 @@ impl StoreLock {
     fn acquire() -> Result<Self, String> {
         let path = lock_path();
         let file = fs::OpenOptions::new()
-            .create(true).write(true).open(&path)
+            .create(true).truncate(false).write(true).open(&path)
             .map_err(|e| format!("open lock {}: {}", path.display(), e))?;
 
         // Blocking exclusive lock
@@ -268,7 +268,7 @@ pub struct GapRecord {
 }
 
 // The full in-memory store
-#[derive(Serialize, Deserialize)]
+#[derive(Default, Serialize, Deserialize)]
 pub struct Store {
     pub nodes: HashMap<String, Node>,          // key → latest node
     #[serde(skip)]
@@ -279,18 +279,6 @@ pub struct Store {
     pub params: Params,
 }
 
-impl Default for Store {
-    fn default() -> Self {
-        Store {
-            nodes: HashMap::new(),
-            uuid_to_key: HashMap::new(),
-            relations: Vec::new(),
-            retrieval_log: Vec::new(),
-            gaps: Vec::new(),
-            params: Params::default(),
-        }
-    }
-}
 
 impl Store {
     /// Load store: try state.json cache first, rebuild from capnp logs if stale
@@ -498,7 +486,7 @@ impl Store {
 
     /// Add a relation (appends to log + updates cache)
     pub fn add_relation(&mut self, rel: Relation) -> Result<(), String> {
-        self.append_relations(&[rel.clone()])?;
+        self.append_relations(std::slice::from_ref(&rel))?;
         self.relations.push(rel);
         Ok(())
     }
@@ -1036,7 +1024,7 @@ impl Store {
         let mut output = String::new();
         for node in &sections {
             if node.key.contains('#') {
-                let section_id = node.key.split('#').last().unwrap_or("");
+                let section_id = node.key.rsplit_once('#').map_or("", |(_, s)| s);
 
                 let links: Vec<_> = self.relations.iter()
                     .filter(|r| r.source_key == node.key && !r.deleted