fix(snippets): read the stored provenance before writing the fresh stamp into the response

fields aliases data['snippet'], so stamping the response first made the
staleness check compare the new stamp to itself — the background persist
never fired. Caught by test_current_code_confirms_and_refreshes_provenance
on run 3811, which exists for exactly this write.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/services/snippets.py

@@ -1049,13 +1049,18 @@ async def attach_live_body(note, data: dict) -> None:
         data["body_source"] = "forge"
         data["body_freshness"] = "current"
         if fetched.commit_sha:
+            # Read the stored stamp BEFORE writing the fresh one into the
+            # response: `fields` aliases data["snippet"], so the other order
+            # makes the staleness check compare the new stamp to itself and
+            # the persist never fires (caught by the unit test, run 3811).
+            stored_prov = fields.get("provenance") or {}
+            stale = stored_prov.get("commit_sha") != fetched.commit_sha
             prov = compose_provenance(commit_sha=fetched.commit_sha)
             # Reflected in THIS response as well as persisted — the reader
             # shouldn't need a second pull to see the stamp they caused.
             if isinstance(data.get("snippet"), dict):
                 data["snippet"]["provenance"] = prov
-            stored_prov = fields.get("provenance") or {}
-            if stored_prov.get("commit_sha") != fetched.commit_sha:
+            if stale:
                 spawn(
                     _refresh_provenance(note, fetched.commit_sha),
                     site="pull provenance-refresh",