Cite a repeated chunk from its last occurrence

Collision detection replaced the lookup's dict comprehension with
`setdefault`, which also flipped a chunk found by several searches from its
last occurrence to its first. The copies differ in everything the expansion
window decides, figures included, so that silently changed what a citation
renders.

The rules are separate now: a repeated (source, chunk_id) takes the later
result, a chunk_id under two sources is still refused.

haiku_rag_slim/haiku/rag/store/models/citation.py

@@ -74,19 +74,23 @@ def resolve_citations(
 ) -> list[Citation]:
     """Resolve chunk IDs to full Citation objects with metadata.
 
-    Raises ``AmbiguousCitationError`` when a cited id names a chunk in more than
-    one of the databases searched, as after copying a database. A citation
-    records the id alone, so resolving one would attribute the answer to a
-    database it may not have come from.
+    A chunk returned by more than one search resolves to its last occurrence.
+    Raises ``AmbiguousCitationError`` instead when a cited id names a chunk in
+    more than one of the databases searched, as after copying a database: a
+    citation records the id alone, so resolving one would attribute the answer
+    to a database it may not have come from.
     """
     by_id: dict[str, SearchResult] = {}
     ambiguous: dict[str, set[str | None]] = {}
     for r in search_results:
         # A result built by hand carries no id and nothing can cite it.
         if cid := r.chunk_id:
-            held = by_id.setdefault(cid, r)
-            if held.source != r.source:
+            if (held := by_id.get(cid)) is not None and held.source != r.source:
                 ambiguous.setdefault(cid, {held.source}).add(r.source)
+            # A chunk found by several searches is expanded once per search, so
+            # the copies differ in content, window and figures. The later entry
+            # wins.
+            by_id[cid] = r
 
     citations = []
     for raw_id in cited_chunk_ids:

tests/multi_db/test_citations.py

@@ -66,7 +66,9 @@ class TestSharedChunkIds:
 
     def test_a_repeated_id_from_one_database_still_collapses(self):
         """One database cannot hold two chunks under one id, so seeing it twice
-        is the same chunk seen twice."""
+        is the same chunk seen twice, and it resolves rather than raising. Which
+        copy supplies the content is `resolve_citations`' own rule, pinned in
+        `tests/store/test_citation.py`."""
         results = [
             SearchResult(
                 content="first",
@@ -88,7 +90,8 @@ class TestSharedChunkIds:
 
         [citation] = resolve_citations(["c1"], results)
 
-        assert citation.content == "first"
+        assert citation.chunk_id == "c1"
+        assert citation.source == "alpha"
 
     def test_only_a_cited_id_has_to_be_unambiguous(self):
         """An id the answer never cites attributes nothing."""

tests/store/test_citation.py

@@ -62,3 +62,41 @@ def test_resolve_citations_copies_chunk_meta():
     result = _result("c1", chunk_meta={"para_no": "12", "speaker": "MR SMITH"})
     citations = resolve_citations(["c1"], [result])
     assert citations[0].chunk_meta == {"para_no": "12", "speaker": "MR SMITH"}
+
+
+def test_a_repeated_chunk_is_cited_from_its_last_occurrence():
+    """One chunk is returned by several searches, each expanded against what
+    that search found in the same document, so the copies differ in everything
+    the window decides. The later entry supplies them."""
+    earlier = SearchResult(
+        content="narrow window",
+        score=0.9,
+        chunk_id="c1",
+        chunk_ids=["c1"],
+        document_id="doc-1",
+        document_uri="test://doc",
+        doc_item_refs=["#/texts/4"],
+        page_numbers=[2],
+        headings=["Maintenance"],
+    )
+    later = SearchResult(
+        content="wider window",
+        score=0.4,
+        chunk_id="c1",
+        chunk_ids=["c1", "c2"],
+        document_id="doc-1",
+        document_uri="test://doc",
+        doc_item_refs=["#/texts/4", "#/texts/5", "#/pictures/0"],
+        page_numbers=[2, 3],
+        headings=["Maintenance", "Calibration"],
+    )
+
+    [citation] = resolve_citations(["c1"], [earlier, later])
+
+    assert citation.content == "wider window"
+    assert citation.chunk_ids == ["c1", "c2"]
+    assert citation.doc_item_refs == ["#/texts/4", "#/texts/5", "#/pictures/0"]
+    assert citation.page_numbers == [2, 3]
+    assert citation.headings == ["Maintenance", "Calibration"]
+    # The window decides which figures travel with the citation.
+    assert citation.picture_refs == ["#/pictures/0"]