fix(evidence): reconcile generations safely

harness/tests/test_corpus_pipeline.py

@@ -61,6 +61,12 @@ class Vectors:
         ]
         return 0
 
+    def list_evidence_generations(self, collection):
+        return sorted({
+            value.record.metadata["vector_generation"] for value in self.records
+            if value.record.kind == "evidence"
+        })
+
 
 class InterruptingVectors(Vectors):
     def __init__(self):
@@ -142,6 +148,22 @@ def test_retention_keeps_filesystem_when_vector_purge_fails_then_retries(tmp_pat
     assert len(CorpusStore(tmp_path / "corpus").list_generations()) == 1
 
 
+def test_gc_reconciles_vector_only_generation(tmp_path):
+    vectors = Vectors()
+    orphan = "gen:" + "f" * 32
+    from tht.ports.vector import VectorWriteRecord
+    from tht.vectorstore.records import VectorRecord
+    vectors.records.append(VectorWriteRecord(
+        record=VectorRecord(id="orphan", kind="evidence", ref="doc:x", title="", content="x",
+                            metadata={"vector_generation": orphan}),
+        embedding=[0.0, 0.0, 0.0], content_hash="sha256:" + "0" * 64,
+    ))
+    candidate = pipeline(tmp_path, Source([]), vectors=vectors, retain=1)
+    report = candidate.gc(workspace_root=tmp_path)
+    assert report["evicted"] == [orphan]
+    assert vectors.list_evidence_generations("evidence") == []
+
+
 def test_unchanged_documents_skip_acquire_normalize_chunk_and_embed(tmp_path):
     one = item("one", "a")
     first_source = Source([(one, "hello")])