Function find 

pub fn find(
    indices: &[Value],
    ranking_map: &Value,
    scores_map: &Value,
    epoch: u64,
) -> Result<Option<Snapshot>, Box<dyn Error>>

Find the snapshot for a given epoch, checking scores.json first (v3), then falling back to ranking.json (v2).

Returns:

• Ok(None) if no prior index exists (first commit, no ranking needed)
• Ok(Some(snapshot)) if found in either source
• Err(...) if a prior index exists but its commit hash is missing from both sources (stale cache)