Issue 20 similarity-index plugins closeout

Status: complete. All 12 recorded slices are complete (2026-08-25).

Delivered backend-neutral similarity-index contracts, core exact search, FAISS flat/HNSW candidates, durable-vector ownership, profile routing, lifecycle operations and installer/bundle integration. The coverage audit records implementation commits fd8d927 and a36771e, operator-journey commit 2c997d0, and completion of the final characterization and branch gate.

Current contracts are in ADR-031 and storage backends. The characterization report retains the measured corpus and reproduction context. Observed flat/HNSW recall@10 of 1.0000 on 128 vectors/16 queries is a non-gating observation.

The recorded final gate exited 0 with formatting, mypy, zero Semgrep findings, pytest, strict docs, installed-wheel rehearsal, catalog, index and audit checks. Issue acceptance evidence explicitly excludes #59; Qdrant was a separate follow-up. Neither this closeout nor those synthetic measurements establish general retrieval quality.

Full execution record

The approved decisions, implementation plan and detailed evidence remain in Git at the pre-cleanup snapshot. Recover this document's complete ledger with:

git show f7d27c8b66777a5d0ec0b5bd231e4ddedb36ec42:docs/process/issue-020-similarity-index-plugins-execution-ledger.md

This closeout replaces the completed execution plan after the operator-approved 2026-10-07 cleanup. It does not authorize new implementation or external actions.