fix(s31): close 1 review BLOCKER — per-call retriever + run_portfolio forwards semantic_retrieval

src/portfolio_optimiser/verdicts.py

@@ -264,20 +264,30 @@ class VerdictStore:
         that want a fresh store straight from a folder."""
         return cls(verdicts=load_verdicts_from_dir(directory))
 
-    def retrieve(self, query: ProposalFeatures, k: int) -> list[Verdict]:
+    def retrieve(
+        self,
+        query: ProposalFeatures,
+        k: int,
+        *,
+        retriever: semretrieval.Retriever | None = None,
+    ) -> list[Verdict]:
         """Return the top-``k`` verdicts. Deterministic: ties break by verdict id, so ordering
         is stable across runs.
 
-        Ranking is delegated to ``self.retriever``, defaulting to ``StructuralRetriever`` — the
-        same weighted structural score and ``(-similarity, id)`` key as before the seam existed.
-        A caller that installs a ``HybridRanker`` opts into an additional semantic term."""
+        Ranking resolves in three steps: the explicit ``retriever`` argument, then
+        ``self.retriever``, then ``StructuralRetriever`` — the same weighted structural score and
+        ``(-similarity, id)`` key as before the seam existed.
+
+        ``retriever`` is PER CALL and keyword-only. It exists because the alternative — assigning
+        ``store.retriever`` — mutates an object the caller owns, so an opt-in made for one
+        retrieval silently governed every later use of that store (including a subsequent run with
+        the flag OFF). ``self.retriever`` survives as the store-level default for callers that
+        genuinely want a store to rank one way for its whole lifetime; see ``docs/extending.md``."""
         if k <= 0:
             raise ValueError(f"k must be positive, got {k}")
-        ranker = (
-            self.retriever
-            if self.retriever is not None
-            else semretrieval.StructuralRetriever(similarity)
-        )
+        ranker = retriever if retriever is not None else self.retriever
+        if ranker is None:
+            ranker = semretrieval.StructuralRetriever(similarity)
         return ranker.rank(query, self.verdicts, k)
 
     def add(self, verdict: Verdict) -> None:
@@ -298,14 +308,24 @@ class ExpeLContextProvider(ContextProvider):
     """Wraps ``VerdictStore.retrieve`` for ExpeL few-shot injection via the GA
     ``ContextProvider`` hook."""
 
-    def __init__(self, store: VerdictStore, query: ProposalFeatures, *, k: int = 3) -> None:
+    def __init__(
+        self,
+        store: VerdictStore,
+        query: ProposalFeatures,
+        *,
+        k: int = 3,
+        retriever: semretrieval.Retriever | None = None,
+    ) -> None:
         super().__init__(source_id="expel-verdictstore")
         self._store = store
         self._query = query
         self._k = k
+        # Per-call ranker, threaded straight through to ``retrieve`` — see its docstring for why
+        # this is a parameter rather than an assignment on the store.
+        self._retriever = retriever
 
     def format_fewshot(self) -> str:
-        hits = self._store.retrieve(self._query, self._k)
+        hits = self._store.retrieve(self._query, self._k, retriever=self._retriever)
         body = "\n".join(f"- [{v.id}] {v.decision}: {v.rationale}" for v in hits)
         return f"Relevant prior verdicts (ExpeL few-shot):\n{body}"