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>

src/core/search.py

@@ -0,0 +1,88 @@
+"""语义检索模块."""
+from src.core.db import VectorDB
+from src.core.embedder import Embedder
+
+
+class Searcher:
+    """向量检索器."""
+
+    def __init__(self, db: VectorDB, embedder: Embedder, collection_name: str):
+        self.db = db
+        self.embedder = embedder
+        self.collection_name = collection_name
+
+    @property
+    def collection(self):
+        return self.db.get_or_create_collection(self.collection_name)
+
+    def search(
+        self,
+        query: str,
+        top_k: int = 10,
+        source_file: str | None = None,
+    ) -> list[dict]:
+        """语义检索, 返回格式化结果列表."""
+        query_embedding = self.embedder.embed([query])[0]
+
+        where_filter = None
+        if source_file:
+            where_filter = {"source_file": source_file}
+
+        results = self.collection.query(
+            query_embeddings=[query_embedding],
+            n_results=top_k,
+            where=where_filter,
+            include=["documents", "metadatas", "distances"],
+        )
+
+        formatted = []
+        if results["ids"] and results["ids"][0]:
+            for i, doc_id in enumerate(results["ids"][0]):
+                metadata = results["metadatas"][0][i] if results["metadatas"] else {}
+                distance = results["distances"][0][i] if results["distances"] else 0.0
+                # ChromaDB 默认用余弦距离, 转为相似度分数 (0~1)
+                score = round(1.0 - distance, 4)
+
+                formatted.append({
+                    "id": doc_id,
+                    "content": results["documents"][0][i] if results["documents"] else "",
+                    "source_file": metadata.get("source_file", ""),
+                    "section_title": metadata.get("section_title", ""),
+                    "heading_level": metadata.get("heading_level", 0),
+                    "chunk_index": metadata.get("chunk_index", 0),
+                    "score": max(0.0, score),
+                })
+
+        return formatted
+
+    def get_collection_info(self) -> dict:
+        """获取 collection 信息."""
+        return {
+            "name": self.collection_name,
+            "count": self.collection.count(),
+        }
+
+    def list_sources(self) -> list[str]:
+        """列出所有已入库的源文件."""
+        if self.collection.count() == 0:
+            return []
+        result = self.collection.get(include=["metadatas"])
+        sources = set()
+        if result and result["metadatas"]:
+            for m in result["metadatas"]:
+                if m and "source_file" in m:
+                    sources.add(m["source_file"])
+        return sorted(sources)
+
+    def delete_by_source(self, file_name: str) -> bool:
+        """按文件名删除文档."""
+        try:
+            existing = self.collection.get(
+                where={"source_file": file_name}
+            )
+            if existing and existing["ids"]:
+                self.collection.delete(ids=existing["ids"])
+                return True
+        except Exception:
+            pass
+        return False