feat: add semantic search module

实现语义检索模块 Searcher 类，支持：
- search(): 向量语义检索，返回格式化结果（含 content、source_file、section_title、heading_level、chunk_index、score 字段）
- get_collection_info(): 获取 collection 统计信息
- list_sources(): 列出已入库源文件
- delete_by_source(): 按源文件删除文档

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

tests/test_search.py

@@ -0,0 +1,76 @@
+"""检索模块测试."""
+import tempfile
+
+import pytest
+
+from src.core.config import EmbedConfig
+from src.core.db import VectorDB
+from src.core.embedder import create_embedder
+from src.core.ingest import DocumentIngestor
+from src.core.search import Searcher
+
+
+@pytest.fixture
+def searcher():
+    """创建带测试数据的 Searcher."""
+    tmpdir = tempfile.mkdtemp()
+    db = VectorDB(persist_dir=tmpdir)
+    embedder = create_embedder(EmbedConfig(mode="local"))
+    ingestor = DocumentIngestor(db, embedder, "test_search")
+
+    # 入库一些测试文档
+    content = """# Python 入门
+Python 是一种解释型编程语言。
+
+## 安装 Python
+从 python.org 下载安装包。
+
+# 向量数据库
+ChromaDB 是一个轻量级向量数据库。
+
+## ChromaDB 安装
+使用 pip install chromadb 安装。"""
+    ingestor.ingest_content(content, "guide.md")
+
+    return Searcher(db, embedder, "test_search")
+
+
+class TestSearcher:
+    """检索器测试."""
+
+    def test_search_returns_results(self, searcher):
+        """搜索返回至少一条结果."""
+        results = searcher.search("Python 编程", top_k=3)
+        assert len(results) > 0
+        for r in results:
+            assert r["content"]
+            assert r["source_file"]
+            assert "score" in r
+
+    def test_search_scores_are_descending(self, searcher):
+        """搜索结果按相似度降序排列."""
+        results = searcher.search("向量数据库", top_k=5)
+        scores = [r["score"] for r in results]
+        assert scores == sorted(scores, reverse=True)
+
+    def test_search_respects_top_k(self, searcher):
+        """top_k 参数限制返回数量."""
+        results = searcher.search("安装", top_k=2)
+        assert len(results) <= 2
+
+    def test_search_returns_all_fields(self, searcher):
+        """搜索结果包含完整字段."""
+        results = searcher.search("ChromaDB", top_k=1)
+        if results:
+            r = results[0]
+            assert "content" in r
+            assert "source_file" in r
+            assert "section_title" in r
+            assert "heading_level" in r
+            assert "chunk_index" in r
+            assert "score" in r
+
+    def test_search_no_results(self, searcher):
+        """无语义匹配时不崩溃."""
+        results = searcher.search("xyzxyz不存在的内容abcabc", top_k=3)
+        assert isinstance(results, list)