feat(search): add score_kind to VectorRetriever (Cosine) and hybrid test helpers (Rrf)

This commit unblocks Tasks 3 and 4 of fb-38:
- VectorRetriever::build_hit now labels hits with ScoreKind::Cosine
- Hybrid retriever test helpers (mk_hit functions) label synthetic hits with ScoreKind::Rrf
- Updated lexical snapshot fixture to reflect new score_kind field in output

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

crates/kebab-search/tests/fixtures/search/lexical/run-1.json

@@ -26,6 +26,7 @@
       "vector_rank": null,
       "vector_score": null
     },
+    "score_kind": "bm25",
     "section_label": "Snap",
     "snippet": "alpha alpha",
     "stale": false
@@ -57,6 +58,7 @@
       "vector_rank": null,
       "vector_score": null
     },
+    "score_kind": "bm25",
     "section_label": "Snap",
     "snippet": "alpha bravo charlie",
     "stale": false