fix: deterministic fusion tie-order + seeded random eval sampling

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/bin/alpha_id.rs

@@ -37,12 +37,20 @@ enum Cmd {
 
 fn run_eval(p: &Pipeline, m: Mode, cases: usize, seed: u64) -> (f64, f64) {
     use alpha_id::eval::{inject_errors, recall_at_k};
-    let billable: Vec<_> = p.entries.iter().enumerate()
+    use rand::SeedableRng;
+    use rand::seq::SliceRandom;
+    let all_billable: Vec<_> = p.entries.iter().enumerate()
         .filter(|(_, e)| e.valid)
         .filter(|(_, e)| alpha_id::corpus::primary_code(e)
             .and_then(|c| p.meta.get(c))
             .map(|mt| mt.para295 == "P").unwrap_or(false))
-        .take(cases).collect();
+        .collect();
+    let mut rng = rand::rngs::StdRng::seed_from_u64(seed);
+    let n = cases.min(all_billable.len());
+    let billable: Vec<_> = all_billable
+        .choose_multiple(&mut rng, n)
+        .cloned()
+        .collect();
     let mut sum = 0.0;
     for (k, (_, e)) in billable.iter().enumerate() {
         let code = alpha_id::corpus::primary_code(e).unwrap().to_string();