Fix systemd boot runtime supervision

[wnlen]

Summary

• Change systemd and systemd-user units to Type=simple and run the kernel in the foreground through clashctl run-direct.
• Keep the existing background start-direct path for script/manual compatibility.
• Refresh the runtime backend entry during clashctl update so existing installs receive the new unit template.
• Expand clashctl logs service output with systemctl status, journal, and mihomo file log fallback.
• Add a regression check that systemd units no longer use Type=forking/PIDFile.

Validation

• bash -n install.sh uninstall.sh scripts/core/*.sh scripts/init/*.sh scripts/dev/*.sh
• for f in scripts/dev/*.sh; do bash "$f"; done

Issue

Addresses #287.