fix: anchor stopwords - remove generic question patterns causing cross-topic contamination

- Add ANCHOR_STOPWORDS set in anchor.py (真正通用的疑问pattern)
- Filter Chinese n-grams against stopwords in extract()
- Update sparse.py content_words extraction to use stopword-filtered query
- Diagnosis: 'Git rebase vs merge' query now correctly excludes Redis/asyncio blocks
- Phase1 results: Full CGK 42.6 tokens avg, 0% contamination (vs Last-5 67.6 tokens, 100%)
- Phase2 ablation: Gate-only accounts for most of the benefit
- Phase3 sensitivity: OVERLAP/NEW_RATIO thresholds insensitive on clean data;
  RECENT_WINDOW is the primary token budget control

Known honest limitations:
- Test set is clean 4-topic synthetic data (no real dirty dialogue)
- No strong baselines (BM25 ablation incomplete)
- No answer-level evaluation (only retrieval blocks measured)
- No parameter sensitivity on noisy real-world data
- Zero contamination on 5 queries is not generalizable

experiments/diagnose_contamination.py

@@ -0,0 +1,88 @@
+#!/usr/bin/env python3
+"""诊断：为什么 Full CGK 有 20% 污染率"""
+import sys, os
+sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))
+
+from src.gatekeeper import ContextGatekeeper
+
+redis_qa = [
+    ("Redis 分布式锁和 RedLock 算法有什么区别？", "RedLock是..."),
+    ("Redis 集群环境下怎么做分布式锁？", "集群下..."),
+    ("Redis 惰性删除和定期删除有什么区别？", "惰性删除..."),
+    ("Redis 的过期 key 对 RDB 快照有什么影响？", "过期key..."),
+    ("Redis 主从复制断线后如何增量同步？", "PSYNC..."),
+]
+asyncio_qa = [
+    ("asyncio.Task 的 cancel 方法怎么工作的？", "cancel..."),
+    ("asyncio.gather 和 asyncio.wait 的返回结果有什么区别？", "gather..."),
+    ("asyncio 的事件循环怎么启动和停止？", "事件循环..."),
+    ("asyncio.sleep 和 time.sleep 的区别是什么？", "sleep..."),
+    ("asyncio 的 Future 对象怎么获取结果？", "Future..."),
+]
+pg_qa = [
+    ("PostgreSQL 的 MVCC 机制是怎么保证读不阻塞写的？", "MVCC..."),
+    ("PostgreSQL 的 VACUUM 为什么要定期运行？", "VACUUM..."),
+    ("EXPLAIN ANALYZE 怎么看执行计划？", "EXPLAIN..."),
+    ("PostgreSQL B-tree 索引和 Hash 索引的区别是什么？", "B-tree..."),
+    ("PostgreSQL 的 TOAST 机制是什么？", "TOAST..."),
+]
+git_qa = [
+    ("Git 的 rebase 和 merge 的区别是什么？", "rebase..."),
+    ("Git reset 的 --soft、--mixed、--hard 有什么区别？", "reset..."),
+    ("Git stash 暂存区和工作目录的区别是什么？", "stash..."),
+    ("Git 的 bisect 怎么用来快速定位 bug？", "bisect..."),
+    ("Git 的 reflog 怎么用来恢复误删的提交？", "reflog..."),
+]
+
+def build_gate():
+    g = ContextGatekeeper(token_budget=4000)
+    for i in range(5):
+        g.add_turn(redis_qa[i][0], redis_qa[i][1])
+        g.add_turn(asyncio_qa[i][0], asyncio_qa[i][1])
+        g.add_turn(pg_qa[i][0], pg_qa[i][1])
+        g.add_turn(git_qa[i][0], git_qa[i][1])
+    return g
+
+def diagnose(query, target_topic):
+    gate = build_gate()
+
+    print(f"\n{'='*60}")
+    print(f"Query: {query}")
+    print(f"Target: {target_topic}")
+    print(f"="*60)
+
+    # 提取 query 锚点
+    q_anchors, has_deictic = gate.anchor_extractor.extract_with_deictic(query)
+    print(f"Query anchors: {q_anchors}")
+    print(f"Has deictic: {has_deictic}")
+
+    # 话题切换检测
+    switched = gate.topic_gate.is_topic_switch(query, gate._active_topic)
+    print(f"Topic switched: {switched}")
+
+    # 召回的块
+    sel = gate.select(query)
+    print(f"Selected blocks: {len(sel)}")
+
+    for item in sel:
+        content = item['user'] + item['assistant']
+        found_topics = []
+        for t in ['Redis', 'asyncio', 'PostgreSQL', 'Git']:
+            if t.lower() in content.lower():
+                found_topics.append(t)
+        print(f"  turn {item['turn_id']}: {found_topics} -> {content[:60]}")
+
+    # 检查污染
+    all_text = ' '.join(item['user'] + item['assistant'] for item in sel)
+    other = [t for t in ['Redis','asyncio','PostgreSQL','Git']
+             if t.lower() in all_text.lower() and t.lower() != target_topic.lower()]
+    print(f"Other topics in context: {other}")
+    print(f"IS CONTAMINATED: {len(other) > 0}")
+
+# 诊断那两个污染案例
+diagnose("Git 的 rebase 和 merge 有什么区别？", "Git")
+diagnose("asyncio.Task 的 cancel 方法怎么工作的？", "asyncio")
+
+# 对比：干净的例子
+diagnose("Redis 惰性删除和定期删除有什么区别？", "Redis")
+diagnose("再问Git", "Git reset 和 revert 的应用场景有什么区别？")