Fix spawn batch bug (bevyengine#22672)

# Objective

bevyengine#18670 introduced a small but fatal issue with batch spawning. Previous
tests did not catch this bug because they tested `alloc` and
`alloc_many` separately.

## Solution

- Fix an off by one math mistake in `FreeBufferIterator::next`.
- Fix the part where you need to `ptr.add(index)` if you want the
element at `index`. Whoops.
- Add a test to catch these issues next time

## Testing

- CI
- One new test
- This was originally found
[here](https://discord.com/channels/691052431525675048/749335865876021248/1464359124950319114),
and the reproduction crashes on main but is fine on this branch.

Author: ElliottjPierce

crates/bevy_ecs/src/entity/remote_allocator.rs

@@ -139,7 +139,7 @@ impl Chunk {
         let head = self.first.load(Ordering::Relaxed);
 
         // SAFETY: Caller ensures we are init, so the chunk was allocated via a `Vec` and the index is within the capacity.
-        unsafe { core::slice::from_raw_parts(head, len) }
+        unsafe { core::slice::from_raw_parts(head.add(index as usize), len) }
     }
 
     /// Sets this entity at this index.
@@ -339,7 +339,10 @@ impl<'a> Iterator for FreeBufferIterator<'a> {
         }
 
         let still_need = self.future_buffer_indices.len() as u32;
-        let next_index = self.future_buffer_indices.next()?;
+        if still_need == 0 {
+            return None;
+        }
+        let next_index = self.future_buffer_indices.start;
         let (chunk, index, chunk_capacity) = self.buffer.index_in_chunk(next_index);
 
         // SAFETY: Assured by `FreeBuffer::iter`
@@ -1115,4 +1118,32 @@ mod tests {
         entities.dedup();
         assert_eq!(pre_len, entities.len());
     }
+
+    /// Bevy's allocator doesn't make guarantees about what order entities will be allocated in.
+    /// This test just exists to make sure allocations don't step on each other's toes.
+    #[test]
+    fn allocation_order_correctness() {
+        let mut allocator = Allocator::new();
+        let e0 = allocator.alloc();
+        let e1 = allocator.alloc();
+        let e2 = allocator.alloc();
+        let e3 = allocator.alloc();
+        allocator.free(e0);
+        allocator.free(e1);
+        allocator.free(e2);
+        allocator.free(e3);
+
+        let r0 = allocator.alloc();
+        let mut many = allocator.alloc_many(2);
+        let r1 = many.next().unwrap();
+        let r2 = many.next().unwrap();
+        assert!(many.next().is_none());
+        drop(many);
+        let r3 = allocator.alloc();
+
+        assert_eq!(r0, e3);
+        assert_eq!(r1, e1);
+        assert_eq!(r2, e2);
+        assert_eq!(r3, e0);
+    }
 }