Improve Asuswrt-Merlin 3006 DNS/SDN integration and service lifecycle

[maghuro]

Closes #342

Related to #229.

Summary

This updates the maintained v1.x Asuswrt-Merlin integration to coexist safely with shared Merlin hooks, support Merlin 3006 Guest Network Pro / SDN through the supported extension points, and harden ctrld's service lifecycle and self-upgrade behavior.

Shared dnsmasq hook coexistence

• manage only explicit ctrld-owned blocks inside shared hooks;
• preserve unrelated user/addon content, ordering and existing modes;
• migrate the legacy ctrld-generated hook format;
• remove the historical enclosing-script exit 0;
• perform atomic writes with path/content revalidation.

Merlin 3006 / Guest Network Pro / SDN

• integrate through both dnsmasq.postconf and dnsmasq-sdn.postconf;
• verify generated main and SDN dnsmasq configurations after restart;
• reject unresolved conf-file=, conf-dir= and conf-script= includes during verification;
• never publish complete dnsmasq-N.conf SDN replacements;
• retain a compatibility fallback only for main dnsmasq when the supported hook demonstrably did not apply;
• make Merlin NVRAM setup/restore retry-safe without changing other router backends;
• migrate legacy ctrld dnsmasq snapshots using crash-safe, inode-aware ownership state and conservative restoration.

Merlin service lifecycle

• atomically publish and conservatively migrate ctrld.startup;
• preserve installed argv and POSIX shell argument boundaries;
• perform legacy startup migration from the newly launched binary after self-update;
• validate PID ownership through /proc/<pid>/cmdline and signal the same validated PID;
• serialize Merlin lifecycle operations and the actual CLI restart/upgrade stop-cleanup-start transaction;
• publish missing shared hook stubs with no-clobber semantics;
• remove both current quoted and legacy unquoted hook references during uninstall;
• validate dnsmasq process ownership before the diskmon service-event signal path.

Compatibility

This targets the maintained v1.0 line.

The v1.x dnsmasq implementation deliberately retains text/template; the unrelated main-branch html/template change is not included.

Validation

The final combined branch was validated with:

• go test -race ./...
• Staticcheck
• Ubuntu
• macOS
• Windows

All three CI jobs pass on the final combined tree.

The implementation was developed as independently reviewed/staged changes for hook coexistence, DNS/SDN integration and service lifecycle before being combined on the v1.x maintenance line.