compose 传入 SIM_GRADE_CONCURRENCY，.env.example 补上说明

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

compose.yaml

@@ -33,6 +33,7 @@ services:
       SIM_HEAP_LIMIT_MB: ${SIM_HEAP_LIMIT_MB:-128}
       SIM_CPU_LIMIT_SECONDS: ${SIM_CPU_LIMIT_SECONDS:-120}
       SIM_MAX_RUN_MINUTES: ${SIM_MAX_RUN_MINUTES:-60}
+      SIM_GRADE_CONCURRENCY: ${SIM_GRADE_CONCURRENCY:-16}
     # 容器文件系统只读，只有 /tmp（学生工程的临时目录）和数据卷可写
     read_only: true
     tmpfs: