fix(network): harden route ownership and backup rollback

[LucaLin233]

Summary

• require exact managed initcwnd/initrwnd ownership before route cleanup
• make previous-state snapshots rollback as a set on function failure
• distinguish route query failure from an absent default route
• rollback failed restore attempts to the pre-restore state

Validation

• targeted network optimize tests pass locally
• full GitHub Ubuntu 24.04 and Debian 13 checks required before main fast-forward

No real throughput tests, install/restore execution, or host network writes were performed.

[LucaLin233]

Validation complete: Ubuntu 24.04 and Debian 13 checks passed. Closing the CI-only PR; the verified commit will be fast-forwarded directly to main.