fix(ebook-search): skip comment lines in gold query loader and realign tests

load_gold_queries now skips blank and `//` comment lines so the committed
section separator in queries.jsonl no longer breaks dataset/load-test loading.

Update tests left stale by the search refactor (6bc3011):
- pass the now-required rank_constant to reciprocal_rank_fusion
- expect bm25_candidates to receive the full query and drop the removed
  "BM25 query preparation" timing step
- assert reranking is enabled by default

tests/ebook_search/test_core.py

@@ -68,7 +68,7 @@ def test_reciprocal_rank_fusion_combines_vector_and_bm25_rankings() -> None:
         SearchResult(chunk_id=3, text="c", source_title="C", score=2.1, bm25_score=2.1),
     ]
 
-    fused = reciprocal_rank_fusion(vector_results, lexical_results)
+    fused = reciprocal_rank_fusion(vector_results, lexical_results, rank_constant=60)
 
     assert [result.chunk_id for result in fused] == [2, 1, 3]
     assert fused[0].rank_source == "Hybrid"
@@ -146,7 +146,7 @@ def test_reciprocal_rank_fusion_marks_hybrid_source() -> None:
     vector_results = [SearchResult(chunk_id=1, text="a", source_title="A")]
     lexical_results = [SearchResult(chunk_id=2, text="b", source_title="B")]
 
-    fused = reciprocal_rank_fusion(vector_results, lexical_results)
+    fused = reciprocal_rank_fusion(vector_results, lexical_results, rank_constant=60)
 
     assert {result.rank_source for result in fused} == {"Hybrid"}