fix(context): apply rerank on embedding and fts5 backends too

[atoz96]

retrieval.rerank is global config, but _retrieve only runs the rerank stage on the auto/hybrid branch. an operator who pins retrieval.backend to embedding or fts5 and sets retrieval.rerank.enabled: true gets no reranking at all — silently. the setting reads as active in config.yaml and does nothing.

why this looks like an oversight rather than a decision

the embedding branch already carries the comment for exactly this class of bug:

# Parity with the hybrid path: an operator who opted into
# recency gets it regardless of which backend serves the query.
filtered = _maybe_recency(store, hits=filtered)

recency was given parity across branches; rerank was missed at the same seam. nothing in _configured_rerank scopes the setting to a backend — it reads retrieval.rerank and returns (enabled, top_k) for any caller.

the change

same stage in the same order — recency, pages-first, then rerank — on both single-retriever branches.

the substring fall-through is deliberately left alone: it is the last-resort scan after both retrievers came back empty, and it applies none of the three stages today. widening that is a separate decision.

tests

test_context_rerank_applies_on_single_retriever_backends, parametrized over embedding and fts5. it fails on the previous code with the unreranked order:

AssertionError: assert ['c1', 'c2'] == ['c2', 'c1']

verification

pytest tests/ -q --ignore=tests/embeddings   green
mypy src                                     Success: no issues found in 115 source files
ruff check src tests                         All checks passed!

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📥 Commits

Reviewing files that changed from the base of the PR and between 647c34d and a2159ef.

📒 Files selected for processing (2)

• src/vouch/context.py
• tests/test_retrieval_backend.py

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