Function run_stale_context_fixture 

pub async fn run_stale_context_fixture(
    engine: &MnemoEngine,
    agent_id: &str,
) -> Result<AttributionReport>

Canonical MemFail attribution fixture.

Writes the same fact twice — an older write at high importance and a newer write at low importance — then asks the retrieve stage for the fact. The default hybrid ranker returns the older (stale) record on top. Store + summarize probes succeed (both records are in storage with correct content hashes; no consolidation has run), so the harness attributes the failure to retrieve.

The fixture asserts the attribution shape, not the retriever’s quality on this scenario — the v0.4.7 current-fact-resolver (fact_key post-processor on RecallRequest) is the documented opt-in mitigation. The point of the fixture is that the harness can isolate the operation responsible when the failure is observed.