test(rag): mk_hit gains indexed_at + stale stubs (fb-32)

Test helper missed the SearchHit field expansion from fb-32 Task 1.
UNIX_EPOCH + false placeholders consistent with the cross-crate
synthetic-mock pattern (hybrid.rs, vector.rs build_hit Task 4 stub).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-rag/tests/common/mod.rs

@@ -143,6 +143,9 @@ pub fn mk_hit(
         index_version: IndexVersion("test-iv".to_string()),
         embedding_model: None,
         chunker_version: ChunkerVersion("v1".to_string()),
+        // p9-fb-32: synthetic test mock — staleness not exercised here.
+        indexed_at: time::OffsetDateTime::UNIX_EPOCH,
+        stale: false,
     }
 }