Cover lead for crossing target

Co-authored-by: Codex <codex@openai.com>

airdef-mod/src/sim/regress.rs

@@ -1878,3 +1878,24 @@ fn motor_burnout_reduces_speed_and_stops_powered_effects() {
     assert!(!sim.missiles[0].engine_lit());
     assert!(!out.engines.iter().any(|e| e.key == 0x4000_0000 | id));
 }
+
+#[test]
+fn interceptor_lead_depends_on_meeting_geometry() {
+    let fly = |velocity: DVec3| {
+        let mut sim = Sim::new(120);
+        let pos = DVec3::new(0.0, 100.0, 900.0);
+        sim.add_drone(DroneKind::Shahed, pos, DVec3::ZERO, DVec3::new(0.0, 100.0, 5000.0));
+        sim.drones[0].vel = velocity;
+        let (target, id) = (sim.drones[0].id, sim.id());
+        sim.tick = 100;
+        let fix = Some(Fix { pos, vel: velocity, tick: 100, seeker: false });
+        sim.missiles.push(Missile { id, kind: MissileKind::Pac3, pos: DVec3::new(0.0, 100.0, 0.0), vel: DVec3::Z * 3.0, target, sam: 999, age: 100, hold: 0, fix, seen: 0 });
+        sim.step_missiles(&mut Ground::default(), &mut Out::default());
+        sim.missiles[0].vel
+    };
+    let crossing = fly(DVec3::X);
+    let approaching = fly(-DVec3::Z);
+    let receding = fly(DVec3::Z);
+    assert!(crossing.x > 0.0, "crossing target must be led");
+    assert!(approaching.x.abs() < 1e-9 && receding.x.abs() < 1e-9);
+}