Stable reference to one saved, content-addressed graph version.

save_graph/2 returns a reference to the effective canonical document after node configuration defaults have been materialized. Durable execution loads that exact document, validates it against the executing node's installed contracts, and compiles it without adding defaults introduced later.

References are relative to an owner scope. The same value may be saved by multiple tenants, and every storage operation resolves the tenant independently. A reference is therefore an address, not an authorization credential.