// Safe reusable slots. Logical handles are indices, not Goose references.
pub struct SlotPool<T> { pub items: Vec<T>, free: Vec<usize> }
impl<T> SlotPool<T> {
    pub fn new() -> Self { Self { items: Vec::new(), free: Vec::new() } }
    pub fn alloc(&mut self, value: T) -> usize {
        if let Some(i) = self.free.pop() { self.items[i] = value; i }
        else { let i = self.items.len(); self.items.push(value); i }
    }
    pub fn free(&mut self, i: usize) {
        assert!(i < self.items.len());
        assert!(!self.free.contains(&i), "slot already returned");
        self.free.push(i); // does not drop or invalidate the stored T
    }
}