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/search.rs

@@ -89,12 +89,7 @@ pub fn search(query: &str, store: &impl StoreView) -> Vec<SearchResult> {
                 .take(3)
                 .map(|l| {
                     let t = l.trim();
-                    if t.len() > 100 {
-                        let end = t.floor_char_boundary(97);
-                        format!("{}...", &t[..end])
-                    } else {
-                        t.to_string()
-                    }
+                    crate::util::truncate(t, 97, "...")
                 })
                 .collect::<Vec<_>>()
                 .join("\n         ");