Function update_graph 

pub fn update_graph(
    store: &ForgeStore,
    result: &AttributedRunResult,
    eval_id: &str,
    version_id: &str,
    config: &ForgeConfig,
) -> ForgeResult<UpdateResult>

Update the causal graph from an attributed run result.

Invariant I10: Idempotent — same run_hash produces no graph change on replay.