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/digest.rs

@@ -15,6 +15,7 @@ use std::collections::BTreeSet;
 
 // --- Digest level descriptors ---
 
+#[allow(clippy::type_complexity)]
 struct DigestLevel {
     name: &'static str,
     title: &'static str,