Function write_samples_with_store 

pub fn write_samples_with_store(
    store: &dyn CardStore,
    card_id: &str,
    samples: Vec<Value>,
) -> Result<PathBuf, String>

Write per-case samples to {card_id}.samples.jsonl (write-once).

Each samples entry is serialized as one compact JSON line. Fails if a samples file already exists for this card — mirrors the immutability guarantee of Cards themselves.