[v1.x] Preserve existing Merlin dnsmasq.postconf content

[maghuro]

Important

Local staging/review draft in the fork only. Upstream contribution guidelines require an issue before any PR.

Why this v1.x branch

Current router releases are maintained on the v1.x line. The current stable v1.5.7 tag resolves to the v1.0 maintenance branch, while the open v2 release work removes router-specific integrations entirely.

This is the v1.x-targeted port of draft #1.

Summary

• manage only a clearly marked ctrld block inside the shared Asuswrt-Merlin dnsmasq.postconf;
• preserve unrelated user/addon content and its position;
• migrate the legacy ctrld <= 1.5.7 whole-hook format;
• remove the legacy exit 0 behavior so ctrld does not terminate the host hook;
• make synthetic shebang ownership reversible;
• perform shared-hook updates atomically with pre-write revalidation;
• preserve symlink targets and existing mode bits.

The v1.x port deliberately retains the v1.x text/template implementation in the dnsmasq package rather than importing the unrelated main branch change to html/template.

Scope

Main dnsmasq hook coexistence only. SDN/Guest Network Pro lifecycle changes are staged separately.