ADR 0043: Cache PCA history matrix for refits

Date: 2026-02-24

Status: Accepted

Decision

• Maintain an internal float64 history matrix cache inside DimensionReducer to avoid rebuilding PCA sample matrices on every refit.
• Update the cache incrementally on history upserts and evictions; rebuild only on cache invalidation.
• Extend reduce_commit_embeddings(...) with an optional reducer=... parameter to reuse reducer state across repeated calls.
• Cache one reducer per island in MapElitesManager and synchronise the projection after refit alignment.