feat(sense): T-X4 可解释决策留痕三元结构 + 修复 sense live 分流潜伏 bug（采纳 ai-model-router 决策模型）

- store：新增 sense_decisions 表（CREATE IF NOT EXISTS 幂等迁移），
  insert_decision / list_decisions（JSON 字段反序列化），q_hash 不落原文（D-G6）
- grader：全模式落库决策三元——reasons（§8 判定顺序最小完备集）、
  candidate_scores（线性头 probs）、rejected（落选档位+原因）；
  留痕失败静默不影响决策主链路
- 修复：chat_completions 的 sense 分流引用未导入的 settings_store，NameError
  被 except 吞掉导致 D-G7 live 分流从未生效；改为 build_proxy_router 显式注入
  settings_provider（api.py 传 settings_store），缺省 None 行为安全
- 新增 /proxy/admin/sense-decisions 管理面查询端点（sense 未启用返回空集）
- 回归测试：注入 settings_provider 后 x-campus-tier 头出现且 T1 决策经
  管理面可查（修复前该头永远缺失）

pytest 466 passed（T-X6 后 461 + 5）

gateway/sense/store.py

@@ -7,6 +7,7 @@ WAL；全部访问经全局锁 + 每操作新连接（ReviewQueue 模式），
 from __future__ import annotations
 
 import sqlite3
+import json
 import threading
 import time
 from pathlib import Path
@@ -29,6 +30,17 @@ CREATE TABLE IF NOT EXISTS sense_artifacts(
   version TEXT PRIMARY KEY, kind TEXT NOT NULL,
   path TEXT NOT NULL, metrics TEXT NOT NULL, created_ts INTEGER NOT NULL,
   active INTEGER DEFAULT 0);
+
+CREATE TABLE IF NOT EXISTS sense_decisions(
+  id INTEGER PRIMARY KEY, ts INTEGER NOT NULL,
+  request_id TEXT NOT NULL, consumer TEXT NOT NULL,
+  decided_tier TEXT NOT NULL, executed_tier TEXT NOT NULL DEFAULT '',
+  mode TEXT NOT NULL DEFAULT 'collect', fallback INTEGER NOT NULL DEFAULT 0,
+  policy_version TEXT DEFAULT '', q_hash TEXT DEFAULT '',
+  reasons TEXT NOT NULL DEFAULT '[]',
+  candidate_scores TEXT NOT NULL DEFAULT '{}',
+  rejected TEXT NOT NULL DEFAULT '[]');
+CREATE INDEX IF NOT EXISTS idx_dec_ts ON sense_decisions(ts);
 """
 
 
@@ -131,6 +143,41 @@ class SenseStore:
                 "DELETE FROM tier_observations WHERE ts < ?", (int(ts),))
             return cur.rowcount
 
+    # ---------- 决策留痕（T-X4：可解释路由三元结构） ----------
+    def insert_decision(self, rec: Dict[str, Any]) -> None:
+        """写入一条路由决策记录（reasons/candidate_scores/rejected 为 JSON 串）。"""
+        with self._lock, self._connect() as conn:
+            conn.execute(
+                """INSERT INTO sense_decisions
+                (ts, request_id, consumer, decided_tier, executed_tier, mode,
+                 fallback, policy_version, q_hash, reasons, candidate_scores,
+                 rejected)
+                VALUES (?,?,?,?,?,?,?,?,?,?,?,?)""",
+                (int(rec.get("ts", time.time())), rec["request_id"],
+                 rec["consumer"], rec["decided_tier"],
+                 rec.get("executed_tier", ""), rec.get("mode", "collect"),
+                 1 if rec.get("fallback") else 0,
+                 rec.get("policy_version", ""), rec.get("q_hash", ""),
+                 rec.get("reasons", "[]"), rec.get("candidate_scores", "{}"),
+                 rec.get("rejected", "[]")))
+
+    def list_decisions(self, limit: int = 50) -> List[Dict[str, Any]]:
+        """最近 N 条决策（新在前；JSON 字段反序列化为对象）。"""
+        with self._lock, self._connect() as conn:
+            rows = conn.execute(
+                "SELECT * FROM sense_decisions ORDER BY id DESC LIMIT ?",
+                (max(1, int(limit)),)).fetchall()
+        out: List[Dict[str, Any]] = []
+        for r in rows:
+            d = dict(r)
+            for f in ("reasons", "candidate_scores", "rejected"):
+                try:
+                    d[f] = json.loads(d.get(f) or ("{}" if f == "candidate_scores" else "[]"))
+                except json.JSONDecodeError:
+                    pass
+            out.append(d)
+        return out
+
     # ---------- 工件 ----------
     def register_artifact(self, version: str, kind: str, path: str,
                           metrics: Dict[str, Any], active: bool = False,