给 trigger_rule_codes 加 JSON 多值索引；登记 P3 处理结果（docs/25 P3 #21）

实测确认 #21 描述准确：fin_risk_alert 只有 11 个普通 BTREE 索引 + 主键 + alert_no
唯一键，规则命中的 JSON_CONTAINS 查询 EXPLAIN 为 type=ALL、possible_keys=NULL，
即全表扫。MySQL 8.0.27 支持多值索引，故新增迁移 20260911_risk_rule_index：

    ADD INDEX idx_fin_risk_alert_trigger_rule_codes
      ((CAST(`trigger_rule_codes` AS CHAR(16) ARRAY)))

迁移幂等（先查 information_schema.STATISTICS），upgrade/downgrade 往返已验证。
生效后 EXPLAIN 变为 access_type=range 且 key 命中该索引，原始证据留档在
docs/evidence/risk-index-probe.json（由 tools/probe_risk_index.py 生成，只读探查）。

只解决一半，另一半如实记为限制：若干 like(f"%{keyword}%") 全表扫无法用 B-tree 索引，
根治需全文索引 + 中文分词组件（部署依赖），本轮不做。

revision 名刻意压到 32 字符以内 —— alembic_version.version_num 是 VARCHAR(32)，
超长会在写版本号时报 1406，而 DDL 是非事务的，那时索引已经建好了。

docs/25 追加"P3 处理结果"表，逐条登记 17-25 的状态：#19 是协议级重做（keyset 分页）
不单方面改，#21 部分修复，#25 前半段不成立，其余已修。

docs/evidence/risk-index-probe.json

@@ -0,0 +1,154 @@
+{
+  "mysql_version": "8.0.27",
+  "column": [
+    {
+      "COLUMN_NAME": "trigger_rule_codes",
+      "COLUMN_TYPE": "json",
+      "DATA_TYPE": "json",
+      "IS_NULLABLE": "NO"
+    }
+  ],
+  "indexes": [
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_alert_level",
+      "COLUMN_NAME": "alert_level",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_alert_type",
+      "COLUMN_NAME": "alert_type",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_created_at",
+      "COLUMN_NAME": "created_at",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_customer_id",
+      "COLUMN_NAME": "customer_id",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_due_at",
+      "COLUMN_NAME": "due_at",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_handler_id",
+      "COLUMN_NAME": "handler_id",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_primary_risk_work_order_id",
+      "COLUMN_NAME": "primary_risk_work_order_id",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_priority_score",
+      "COLUMN_NAME": "priority_score",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_related_order_id",
+      "COLUMN_NAME": "related_order_id",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_related_transaction_id",
+      "COLUMN_NAME": "related_transaction_id",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_related_work_order_id",
+      "COLUMN_NAME": "related_work_order_id",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_status",
+      "COLUMN_NAME": "status",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "idx_fin_risk_alert_trigger_rule_codes",
+      "COLUMN_NAME": null,
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": "cast(`trigger_rule_codes` as char(16) array)"
+    },
+    {
+      "INDEX_NAME": "PRIMARY",
+      "COLUMN_NAME": "id",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    },
+    {
+      "INDEX_NAME": "uk_fin_risk_alert_alert_no",
+      "COLUMN_NAME": "alert_no",
+      "INDEX_TYPE": "BTREE",
+      "EXPRESSION": null
+    }
+  ],
+  "row_count": 3,
+  "non_array_rows": 0,
+  "distinct_rule_codes": [
+    {
+      "raw_codes": "[\"RW-007\", \"RW-002\", \"RW-012\"]",
+      "rows_count": 1
+    },
+    {
+      "raw_codes": "[\"RW-015\", \"RW-003\"]",
+      "rows_count": 1
+    },
+    {
+      "raw_codes": "[\"RW-018\"]",
+      "rows_count": 1
+    }
+  ],
+  "explain_json_contains": {
+    "query_block": {
+      "select_id": 1,
+      "cost_info": {
+        "query_cost": "0.71"
+      },
+      "table": {
+        "table_name": "fin_risk_alert",
+        "access_type": "range",
+        "possible_keys": [
+          "idx_fin_risk_alert_trigger_rule_codes"
+        ],
+        "key": "idx_fin_risk_alert_trigger_rule_codes",
+        "used_key_parts": [
+          "cast(`trigger_rule_codes` as char(16) array)"
+        ],
+        "key_length": "67",
+        "rows_examined_per_scan": 1,
+        "rows_produced_per_join": 1,
+        "filtered": "100.00",
+        "cost_info": {
+          "read_cost": "0.61",
+          "eval_cost": "0.10",
+          "prefix_cost": "0.71",
+          "data_read_per_join": "936"
+        },
+        "used_columns": [
+          "id",
+          "trigger_rule_codes",
+          "cast(`trigger_rule_codes` as char(16) array)"
+        ],
+        "attached_condition": "json_contains(cast(`trigger_rule_codes` as char(16) array),json'[\"RW-018\"]')"
+      }
+    }
+  }
+}