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/util.rs

@@ -12,3 +12,19 @@ pub fn memory_subdir(name: &str) -> Result<PathBuf, String> {
         .map_err(|e| format!("create {}: {}", dir.display(), e))?;
     Ok(dir)
 }
+
+/// Truncate text to `max_len` bytes at a char boundary, appending `suffix`.
+/// Returns the original string if it's already short enough.
+pub fn truncate(text: &str, max_len: usize, suffix: &str) -> String {
+    if text.len() <= max_len {
+        text.to_string()
+    } else {
+        let end = text.floor_char_boundary(max_len);
+        format!("{}{}", &text[..end], suffix)
+    }
+}
+
+/// Take the first `n` chars from a string.
+pub fn first_n_chars(s: &str, n: usize) -> String {
+    s.chars().take(n).collect()
+}