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/types.rs

@@ -26,6 +26,7 @@ use std::time::{SystemTime, UNIX_EPOCH};
 macro_rules! capnp_enum {
     ($rust_type:ident, $capnp_type:path, [$($variant:ident),+ $(,)?]) => {
         impl $rust_type {
+            #[allow(clippy::wrong_self_convention)]
             pub(crate) fn to_capnp(&self) -> $capnp_type {
                 match self {
                     $(Self::$variant => <$capnp_type>::$variant,)+

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);
         }
     }