feat: 增加内存观测日志

- bin/maa-online-memlog: 每分钟采集一次服务 cgroup 匿名内存、swap 占用、进程 RSS、
  以及大于 32 MB 的匿名映射数量（glibc arena 膨胀指纹），追加写入 logs/memlog.csv，
  超过 5 MB 自动滚动并保留一代；同时输出一行到 journald
- systemd/maa-online-memlog.service, maa-online-memlog.timer: 每分钟触发采集，
  Nice=10 避免影响流媒体负载
- bin/maa-online-install-systemd: 安装并启用观测定时器，安装提示补充内存日志路径
- README.md, user_config/README.md: 补充内存观测说明、字段表与查看命令
- .gitignore: 忽略运行期产生的 logs/

systemd/maa-online-memlog.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=Sample MAA Online server memory/swap into a CSV log
+
+[Service]
+Type=oneshot
+ExecStart=__ROOT__/bin/maa-online-memlog
+# Pure observation: keep it from competing with the streaming workload.
+Nice=10

systemd/maa-online-memlog.timer

@@ -0,0 +1,12 @@
+[Unit]
+Description=Sample MAA Online server memory every minute
+
+[Timer]
+OnBootSec=1min
+OnUnitActiveSec=1min
+AccuracySec=5s
+Persistent=false
+Unit=maa-online-memlog.service
+
+[Install]
+WantedBy=timers.target