feat(network): finalize automatic bandwidth tuning

[LucaLin233]

Summary

• finalize route-bound IPv4 iperf3 automatic measurement and cache fallback handling
• harden sysctl, BBR, initcwnd, backup, and rollback transactions
• restore the tcpfit v0.5.6 mixed memory model with page-size-aware tcp_mem floors
• retire verify and legacy probe flags while keeping traffic-shape independent

Verification

• bash -n modules/network-optimize.sh
• shellcheck --severity=warning modules/network-optimize.sh
• shellcheck -x --severity=error linux_setup.sh modules/.sh tools/.sh tests/*.sh
• bash tests/run.sh
• Debian 13 VPS: expected and actual tcp_mem both 15280 30560 61120
• Debian 13 VPS: install, status, and restore previous passed; forwarding, IPv6 RA, and ECN remained unchanged