fix: sched round-robin, cap slot OOB panic, clippy, make ci

- RunList -> intrusive FIFO, no starvation, no silent drop
- cap table sized to CapSlot range (kani caught OOB panic)
- snake food color test uses const
- kani cfg lint, RUST_MIN_STACK for big kernel struct in tests

crates/apps/snake/src/tests/renderer.rs

@@ -28,5 +28,5 @@ fn food_is_rendered() {
     let count = render(&snake, &mut commands);
     // Food is command index 2 (after bg and border)
     assert!(count > 2);
-    assert_eq!(commands[2].color, 0xFF0040);
+    assert_eq!(commands[2].color, crate::renderer::FOOD_COLOR);
 }