fix: T7 quality — SynthesisResult.citations tuple, retriever comments, test assertion

- SynthesisResult.citations changed from list[Citation] to tuple[Citation, ...]
  so frozen=True dataclass is genuinely immutable end-to-end
- synthesize() now builds tuple via generator expression
- retriever._combined: add comment explaining L2 distance inversion
- retriever.hybrid_search: comment on _bm25._chunks private access
- test_synthesizer_builds_context_from_chunks: drop vacuous str(call_args)
  fallback; assert directly on call_args.args[0]

app/services/retriever.py

@@ -75,6 +75,7 @@ class Retriever:
                 vector_score=None,
             )
         for vh in vec_hits:
+            # _chunks is the loaded list of dicts from BM25Index; no public accessor exists
             text = next((c["text"] for c in self._bm25._chunks if c["id"] == vh.id), "")
             if vh.id in merged:
                 existing = merged[vh.id]
@@ -98,6 +99,7 @@ class Retriever:
 
         def _combined(r: RetrievedChunk) -> float:
             bm25 = r.bm25_score
+            # sqlite-vec returns L2 distance (lower=better); invert to [0,1] higher-is-better
             vec = (1.0 / (1.0 + r.vector_score)) if r.vector_score is not None else 0.0
             return bm25 * 0.5 + vec * 0.5
 

app/services/synthesizer.py

@@ -28,7 +28,7 @@ class Citation:
 @dataclass(frozen=True)
 class SynthesisResult:
     answer: str
-    citations: list[Citation]
+    citations: tuple[Citation, ...]
 
 
 class Synthesizer:
@@ -47,12 +47,12 @@ class Synthesizer:
 
         answer = self._llm.complete(prompt, system=_SYSTEM_PROMPT)
 
-        citations = [
+        citations = tuple(
             Citation(
                 doc_id=c.doc_id,
                 page_number=c.page_number,
                 snippet=c.text[:200],
             )
             for c in chunks
-        ]
+        )
         return SynthesisResult(answer=answer, citations=citations)

tests/test_synthesizer.py

@@ -40,8 +40,7 @@ def test_synthesizer_builds_context_from_chunks():
     synth = Synthesizer(mock_llm)
     synth.synthesize("Q?", [], [_chunk(text="Detailed rule text here.")])
 
-    call_args = mock_llm.complete.call_args
-    assert "Detailed rule text here." in call_args[0][0] or "Detailed rule text here." in str(call_args)
+    assert "Detailed rule text here." in mock_llm.complete.call_args.args[0]
 
 
 def test_synthesizer_uses_system_prompt():