Constant MAX_COLLECTIONS 

pub const MAX_COLLECTIONS: usize = _; // 1_048_576usize

Maximum number of collections a single catalog may carry. Bounds Catalog::list_collections (Rule 2) and the next-collection-id allocator below. 1 << 20 (1 048 576) is a generous ceiling — at 64-byte descriptor payloads the catalog would still fit in ~64 MiB.