Function rehydrate 

pub async fn rehydrate(
    conn: &Connection,
    ids: &[&str],
    archive_path: &Path,
) -> Result<RehydrateReport>

Move concepts back from the cold file into the hot tables (§2.3, C3).

Rehydration is a move back, not a write

It mints no transaction-time facts and is invisible to both clocks. The concept’s log entries were never removed, so the ledger already says everything true about it; writing a fresh 'I' would assert the concept was learned at rehydration time, and — because the fold takes the highest seq_id per entity — would additionally outrank any later 'U' that retired it. See crate::schema::ddl::CREATE_CONCEPTS_LOG_INSERT, which is marker-gated at v10 for exactly this reason. The whole operation therefore runs inside a declared archive session, which is what suppresses the trigger.

rowid_pk: reinstate, or reassign and re-point the index

The common case has no collision — the rowid was freed by archival and nothing has claimed it since — and reinstating is the clean move-back with no side effects at all. When something has taken it, the fallback is a fresh rowid_pk plus an FTS correction: concepts_fts is external-content keyed on rowid_pk ([D-119]), so a reassignment without re-pointing leaves the index describing the wrong row, silently. Both exits are taken here rather than one being assumed, and RehydrateReport::rowids_reassigned reports which was used.

A concept outlives its lineage, and is refused (0.15.11, W15.1, C-3)

Database::archive_branch takes a lineage’s branches row with it, and a cold concept keeps the branch_id it was minted on. Rehydrating one after the other therefore reinstates a row whose lineage no longer exists — which concepts.branch_id REFERENCES branches(branch_id) refuses, with foreign keys on. It is refused here instead, as DbError::BranchArchived naming both the concept and the lineage, for the reason that variant’s own documentation gives: the engine’s message names neither, and blames the table that was being written rather than the one that is missing.

Ids whose lineage is intact are unaffected — the refusal names one concept, not the batch.

Errors

DbError::BranchArchived when a requested concept’s lineage has no row in branches. Nothing is written: the whole rehydrate runs in one transaction and the refusal leaves it uncommitted.