Constant DECODE_NODES 

const DECODE_NODES: LocalKey<Cell<usize>>;

Cumulative Value nodes decoded so far in the current top-level decode, shared by every array and map in the datum and bounded by MAX_VALUE_NODES. Reset to 0 at each top-level entry (see DecodeDepthGuard::enter) so the budget composes across nesting instead of resetting per collection. Charged via charge_value_nodes.