fix(acorn): clippy clean-up — sort_by_key, is_empty, redundant closures

CI's `Clippy (deny warnings)` flagged three lints introduced by the
previous optimization commit:

- `unnecessary_sort_by` (graph.rs:158, 176) → use `sort_by_key`
- `len_without_is_empty` (graph.rs) → add `AcornGraph::is_empty`
  and `if graph.is_empty()` in search.rs
- `redundant_closure` (main.rs:65, 159, 160) → pass the predicate
  directly to `recall_at_k` instead of `|id| pred(id)`

No semantic change.

Co-Authored-By: claude-flow <ruv@ruv.net>

crates/ruvector-acorn/src/graph.rs

@@ -126,6 +126,10 @@ impl AcornGraph {
         self.data.len() / self.dim.max(1)
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     /// Borrow vector `i` as a contiguous slice — the hot path for L2².
     #[inline(always)]
     pub fn row(&self, i: usize) -> &[f32] {
@@ -155,7 +159,7 @@ pub fn flat_k_nearest(data: &[Vec<f32>], query: &[f32], k: usize) -> Vec<u32> {
         }
     }
     let mut out: Vec<(OrdF32, u32)> = heap.into_sorted_vec();
-    out.sort_by(|a, b| a.0.cmp(&b.0));
+    out.sort_by_key(|a| a.0);
     out.into_iter().map(|(_, id)| id).collect()
 }
 
@@ -173,7 +177,7 @@ pub fn exact_filtered_knn(
         .filter(|&i| predicate(i as u32))
         .map(|i| (OrdF32(l2_sq(&data[i], query)), i as u32))
         .collect();
-    scored.sort_by(|a, b| a.0.cmp(&b.0));
+    scored.sort_by_key(|a| a.0);
     scored.truncate(k);
     scored.into_iter().map(|(_, id)| id).collect()
 }

crates/ruvector-acorn/src/main.rs

@@ -62,7 +62,7 @@ fn run_variant(
     sel_pct: f64,
     predicate: &(dyn Fn(u32) -> bool + Sync),
 ) {
-    let recall = recall_at_k(data, queries, K, |id| predicate(id), index);
+    let recall = recall_at_k(data, queries, K, predicate, index);
     let qps = bench_qps(index, queries, K, predicate);
     let mem_mb = index.memory_bytes() as f64 / 1_048_576.0;
     println!(
@@ -156,8 +156,8 @@ fn main() {
     println!("{}", "-".repeat(44));
     for sel_frac in [0.50, 0.20, 0.10, 0.05, 0.02, 0.01] {
         let pred = selectivity_predicate(N, sel_frac);
-        let r_flat = recall_at_k(&data, &queries, K, |id| pred(id), &flat);
-        let r_acorn = recall_at_k(&data, &queries, K, |id| pred(id), &acorng);
+        let r_flat = recall_at_k(&data, &queries, K, pred, &flat);
+        let r_acorn = recall_at_k(&data, &queries, K, pred, &acorng);
         println!(
             "{:>7.0}%  {:>16.1}%  {:>16.1}%",
             sel_frac * 100.0,

crates/ruvector-acorn/src/search.rs

@@ -34,7 +34,7 @@ pub fn acorn_search(
     ef: usize,
     predicate: impl Fn(u32) -> bool,
 ) -> Vec<(u32, f32)> {
-    if graph.len() == 0 {
+    if graph.is_empty() {
         return vec![];
     }
     let n = graph.len();