feat: 上下文门控器初始实现

- anchor.py: 锚点提取（中文 2/3-gram、英文单词、代码标识符）
- block.py: 对话块数据结构
- topic_gate.py: 话题门控（overlap/new_ratio 判断切换）
- sparse.py: 稀疏召回（BM25/IDF-overlap + exact match 加分）
- selector.py: 最小覆盖贪心选择
- gatekeeper.py: 完整流程封装
- tests/: 单元测试 + 端到端测试（含 MiniMax API 验证）

特性：
- 纯 Python，无额外模型依赖
- 支持 2 核 2G 环境
- 话题门控 + 稀疏召回 + 最小覆盖选择

src/topic_gate.py

@@ -0,0 +1,106 @@
+"""
+话题门控模块 - 判断当前 query 是继续话题还是切换话题
+"""
+
+from typing import List, Tuple
+from .anchor import AnchorExtractor
+
+
+# 指代词列表
+DEICTIC_WORDS = {
+    '这个', '那个', '它', '她', '他', '上面', '刚才', '继续',
+    '展开', '为什么会这样', '怎么改', '然后', '还有', '另外',
+    '这样', '那样', '如此', '前一个', '这一', '那一'
+}
+
+
+class TopicGate:
+    """话题门控：判断是继续当前话题还是切换到新话题"""
+
+    # 阈值参数
+    OVERLAP_CONTINUE_THRESHOLD = 0.45
+    OVERLAP_SWITCH_THRESHOLD = 0.20
+    NEW_RATIO_SWITCH_THRESHOLD = 0.70
+
+    def __init__(self, anchor_extractor: AnchorExtractor = None):
+        self.anchor_extractor = anchor_extractor or AnchorExtractor()
+        self.active_topic_anchors: List[str] = []
+        self.active_topic_idf: dict[str, float] = {}
+
+    def update_active_topic(self, anchors: List[str], idf: dict[str, float]) -> None:
+        """更新活跃话题的锚点集合"""
+        self.active_topic_anchors = anchors
+        self.active_topic_idf = idf
+
+    def is_topic_switch(
+        self, query: str, active_topic: Tuple[List[str], dict[str, float]] = None
+    ) -> bool:
+        """
+        判断当前 query 是否切换了话题
+
+        Returns:
+            True  -> 切换到新话题
+            False -> 继续当前话题
+        """
+        anchors, has_deictic = self.anchor_extractor.extract_with_deictic(query)
+
+        if active_topic is None:
+            self.active_topic_anchors = anchors
+            return False
+
+        topic_anchors, topic_idf = active_topic
+
+        # 有指代词 → 强暗示继续
+        if has_deictic:
+            return False
+
+        # 计算 overlap 和 new_ratio
+        overlap = self._compute_overlap(anchors, topic_anchors, topic_idf)
+        new_ratio = self._compute_new_ratio(anchors, topic_anchors, topic_idf)
+
+        # 切换判断
+        if overlap < self.OVERLAP_SWITCH_THRESHOLD and new_ratio > self.NEW_RATIO_SWITCH_THRESHOLD:
+            return True
+
+        # 继续判断
+        if overlap > self.OVERLAP_CONTINUE_THRESHOLD:
+            return False
+
+        # 中间地带：默认继续
+        return False
+
+    def _compute_overlap(
+        self, query_anchors: List[str], topic_anchors: List[str], topic_idf: dict[str, float]
+    ) -> float:
+        """计算加权重叠率"""
+        if not query_anchors:
+            return 0.0
+
+        overlap_weight = sum(
+            topic_idf.get(a, 1.0)
+            for a in query_anchors
+            if a in topic_anchors
+        )
+        total_weight = sum(topic_idf.get(a, 1.0) for a in query_anchors)
+
+        return overlap_weight / total_weight if total_weight > 0 else 0.0
+
+    def _compute_new_ratio(
+        self, query_anchors: List[str], topic_anchors: List[str], topic_idf: dict[str, float]
+    ) -> float:
+        """计算新锚点比例"""
+        if not query_anchors:
+            return 1.0
+
+        new_anchors = set(query_anchors) - set(topic_anchors)
+        new_weight = sum(topic_idf.get(a, 1.0) for a in new_anchors)
+        total_weight = sum(topic_idf.get(a, 1.0) for a in query_anchors)
+
+        return new_weight / total_weight if total_weight > 0 else 1.0
+
+    def compute_idf_for_anchors(self, anchors: List[str]) -> dict[str, float]:
+        """为锚点集合计算 IDF 值（用于后续计算）"""
+        idf = {}
+        for a in anchors:
+            idf[a] = self.anchor_extractor.idf(a)
+        return idf