Constant DEFAULT_MAX_NODES 

const DEFAULT_MAX_NODES: u32 = _ /* 131_072u32 */;

The number of nodes an allocator tracks unless the caller says otherwise.

This matches the default of the original C++ OffsetAllocator; it costs roughly 3.5 MB of metadata, so an allocator that knows it holds fewer ranges should use Allocator::with_max_nodes_and_alignment.