polish: drop redundant Filter import; document equal eval metrics

src/bin/alpha_id.rs

@@ -37,7 +37,6 @@ enum Cmd {
 
 fn run_eval(p: &Pipeline, m: Mode, cases: usize, seed: u64) -> (f64, f64) {
     use alpha_id::eval::{inject_errors, recall_at_k};
-    use alpha_id::model::Filter;
     let billable: Vec<_> = p.entries.iter().enumerate()
         .filter(|(_, e)| e.valid)
         .filter(|(_, e)| alpha_id::corpus::primary_code(e)
@@ -53,6 +52,8 @@ fn run_eval(p: &Pipeline, m: Mode, cases: usize, seed: u64) -> (f64, f64) {
         sum += recall_at_k(&preds, &code, 10);
     }
     let n = billable.len().max(1) as f64;
+    // Both metrics are equal by construction: the loop runs only over
+    // billable cases, so Recall@10 == BillableRecall@10 here.
     (sum / n, sum / n)
 }