整合求解器活动监控与步长回归证据

同步远端 PNL0003 诊断和大采样网格能力，语义合并活动感知的 60 秒真停滞判定与旧后端 15 分钟兼容兜底。

纳管热路径优化、15 单元运行证据、浏览器与 API 报告，并补充北京时间更新日志和遗留问题。

tests/baselines/simulation/test_mql_full_branches/manifest.json

@@ -129,7 +129,10 @@
     "environment": {
       "SIMULATION_CAUSAL_EXECUTOR_V2": "1",
       "SIMULATION_CAUSAL_COORDINATE_KERNEL": "1",
+      "SIMULATION_CAUSAL_DIRECT_SUM_ASSIGNMENTS": "1",
+      "SIMULATION_CAUSAL_DIRECT_EQUATION_READERS": "1",
       "SIMULATION_CAUSAL_FAST_PATH": "1",
+      "SIMULATION_MECHANICAL_ATOL_MODE": "legacy",
       "SIMULATION_ODE_JACOBIAN_MODE": "scipy",
       "SIMULATIONAPP_PROPERTY_CACHE": "on"
     },