ADR 0009: Immutable file embedding cache (insert-only)

Date: 2026-01-08

Context: Repo-state embeddings rely on a file-level embedding cache keyed by (experiment_id, blob_sha) and on incremental aggregation being stable across time. Decision: MapElitesFileEmbeddingCache entries are immutable values; writes are insert-only and conflicts are ignored (no overwrite of existing vectors). Constraints: Correcting embeddings requires resetting the database schema in development rather than overwriting an existing cache key. Consequences: Concurrent schedulers/workers can safely attempt to insert the same blob without causing vector drift; time-based heuristics are unnecessary for cache correctness.