extract truncation helpers, fix clippy warnings, dedup batching loop

Add util::truncate() and util::first_n_chars() to replace 16 call
sites doing the same floor_char_boundary or chars().take().collect()
patterns. Deduplicate the batching loop in consolidate.rs (4 copies
→ 1 loop over an array). Fix all clippy warnings: redundant closures,
needless borrows, collapsible if, unnecessary cast, manual strip_prefix.

Net: -44 lines across 16 files.

poc-memory/src/store/view.rs

@@ -106,7 +106,7 @@ impl StoreView for MmapView {
     fn for_each_node<F: FnMut(&str, &str, f32)>(&self, mut f: F) {
         let snap = self.snapshot();
         for (key, node) in snap.nodes.iter() {
-            f(&key, &node.content, node.weight);
+            f(key, &node.content, node.weight);
         }
     }