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/neuro/rewrite.rs

@@ -106,10 +106,11 @@ pub fn differentiate_hub_with_graph(store: &Store, hub_key: &str, graph: &Graph)
 
         // Only propose move if there's a reasonable match
         if best_sim > 0.05 && !best_section.is_empty() {
-            let snippet = neighbor_content.lines()
-                .find(|l| !l.is_empty() && !l.starts_with("<!--") && !l.starts_with("##"))
-                .unwrap_or("")
-                .chars().take(80).collect::<String>();
+            let snippet = crate::util::first_n_chars(
+                neighbor_content.lines()
+                    .find(|l| !l.is_empty() && !l.starts_with("<!--") && !l.starts_with("##"))
+                    .unwrap_or(""),
+                80);
 
             moves.push(LinkMove {
                 neighbor_key: neighbor_key.to_string(),