feat(ebook-search): add load-test CLI for the search service

Add a Typer CLI script that drives POST /search on a running server at a
configurable concurrency and reports latency percentiles (p50/p90/p95/p99),
throughput, and HTTP status distribution. Queries are drawn from the shared
eval JSONL set so load testing and evaluation exercise the same questions.

tests/ebook_search/test_dataset.py

@@ -0,0 +1,18 @@
+"""Tests for the shared query/gold set loader."""
+
+from __future__ import annotations
+
+from python.ebook_search.eval.dataset import load_gold_queries
+
+
+def test_default_query_set_counts() -> None:
+    queries = load_gold_queries()
+    answerable = [query for query in queries if query.answerable]
+
+    assert len(queries) == 70
+    assert len(answerable) == 50
+    assert len(queries) - len(answerable) == 20
+    assert all(query.query for query in queries)
+    # Answerable queries carry at least one source; garbage queries carry none.
+    assert all(query.relevant_sources for query in answerable)
+    assert all(not query.relevant_sources for query in queries if not query.answerable)