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

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

tests/eval_compare_tests.rs

@@ -10,3 +10,20 @@ fn eval_mode_c_runs_and_reports_billable_recall() {
     let s = String::from_utf8_lossy(&out.stdout);
     assert!(s.contains("BillableRecall@10="));
 }
+
+#[test]
+fn eval_is_deterministic_same_seed() {
+    let run = || {
+        let out = Command::new(env!("CARGO_BIN_EXE_alpha-id"))
+            .args(["eval", "--mode", "c", "--cases", "40", "--seed", "42"])
+            .arg("--config").arg("config/default.toml")
+            .output().unwrap();
+        assert!(out.status.success(), "stderr: {}", String::from_utf8_lossy(&out.stderr));
+        String::from_utf8(out.stdout).unwrap()
+    };
+    let first = run();
+    let second = run();
+    assert!(!first.is_empty(), "eval produced no output");
+    assert_eq!(first, second,
+        "eval output differed between runs with identical args (seed=42):\n  run1: {first:?}\n  run2: {second:?}");
+}