Function set_graph_cache_default 

pub fn set_graph_cache_default(on: bool)

Set the process-wide default of the graph cache for every store opened AFTER this call (an explicit ENV_GRAPH_CACHE still wins). For an embedder that learns its role at runtime and cannot set the environment — edition 2024 makes set_var unsafe, and gunnar forbids unsafe — such as gunnar’s twin, which holds packs and serves no reads.