Constant CACHE_EPOCH 

pub const CACHE_EPOCH: &str = "schema_v18";

Cache-invalidation epoch — a manual cache-buster folded into the cache key. Bump on any correctness fix that should orphan existing cache files, whether or not the DuckDB schema shape changed. This is deliberately independent of facts::schema::CURRENT_SCHEMA_VERSION (the on-disk schema version): a non-schema correctness fix bumps this without touching the schema version, and the historical schema_v prefix on the value is retained so older cache files stay invalidated.

The current epoch (schema_v18) orphans any cache file that persisted an empty fact store. A shallow / merge-tip checkout ingests zero commits under the default merge filter, and the HEAD-scoped cache key does not fold shallow state — so a run that cached that empty store poisoned the key a later repaired (unshallowed) clone recomputes, turning the ingest-witness failure into a sticky one no re-fetch could clear. Bumping the epoch discards those poisoned entries so the repaired clone rebuilds from full history.

Public so other cache-like artifacts (e.g. codelore diff’s --base-cache) can fold this epoch into their own freshness keys instead of duplicating the literal — see codelore-cli/src/diff.rs::base_cache_opts_digest.