Function backfill 

pub fn backfill(
    repo: &Path,
    records: Vec<MigrationRecord>,
    blame_fallback: Option<&str>,
    jurisdiction_map: &HashMap<String, String>,
    dry_run: bool,
) -> Result<BackfillSummary, String>

Run the idempotent backfill of records into the store at repo. Deterministic order: records are sorted by source_key first so a re-run replays the same chain. Idempotency is keyed on the durable source_key (the non-hashed source_ref’s derived key, or a token in the hashed observe): a record whose key is already in the store is SKIPPED — chain-position-independent, so a re-run over a now-non-empty store writes nothing. The chain is kept by threading the PROSPECTIVE parent (the id we just wrote/found) instead of re-reading the live HEAD each step, so the lineage stays stable across re-runs. A skipped record whose stored parent differs from where it would now land is a back-dated mid-chain insert and is reported as re-linked. blame_fallback supplies the author for a record carrying none; a record with neither is a source-only gap (R5 stays intact — we never invent an author). jurisdiction_map (source_key → A/B/C/D bucket) tags each imported decision: a record whose key is in the map carries that jurisdiction, one absent imports untagged (None) — so the map is purely additive (an empty map ⇒ every record None, the prior behavior). jurisdiction is NON-hashed, so tagging never moves a tick id (idempotency holds across re-runs). --dry-run reports the would-import count but writes nothing.