fix(benchmark): 记录实际Embedding空间与逐样本回退信息

backend/app/retrieval/provenance.py

@@ -0,0 +1,21 @@
+"""Task-local observations of the embedding path actually used by a search."""
+from contextlib import contextmanager
+from contextvars import ContextVar
+
+_observation: ContextVar[dict | None] = ContextVar("embedding_observation", default=None)
+
+
+@contextmanager
+def capture_embedding():
+    result = {"source": "not_used"}
+    token = _observation.set(result)
+    try:
+        yield result
+    finally:
+        _observation.reset(token)
+
+
+def record_embedding(**fields) -> None:
+    result = _observation.get()
+    if result is not None:
+        result.update(fields)