[v1.x] Use Merlin DNS hooks for main and SDN dnsmasq

[maghuro]

Important

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

Stacked on #6 and targeted at the maintained v1.x router-support line. This is the v1.x port of #2.

Summary

Replace the fragile Merlin full-config snapshot model with supported main/SDN hooks and Guest Network Pro handling.

• install ctrld-managed blocks into both dnsmasq.postconf and Merlin 3006 dnsmasq-sdn.postconf;
• verify generated main and SDN dnsmasq configuration after restart;
• fail closed when unresolved conf-file=, conf-dir= or conf-script= includes are present;
• keep a compatibility fallback for main dnsmasq only when the supported hook demonstrably did not apply;
• never create full dnsmasq-N.conf SDN snapshots;
• make NVRAM setup/restore retry-aware only for Merlin;
• ensure the private Merlin state directory exists and its parent-directory entry is durably synced before state is trusted;
• migrate legacy ctrld snapshots with crash-safe inode-aware journaling, private hard-link ownership anchors and conservative restoration;
• protect main fallback publication/cleanup with no-clobber and inode/content revalidation;
• validate stale PID files against the actual ctrld process;
• preserve unrelated shared-hook content and roll back partial hook writes.

Port parity

internal/router/merlin/merlin.go, its regression tests, and internal/router/nvram/nvram.go now match the final #2 implementation. The intentional v1.x difference is that internal/router/dnsmasq/dnsmasq.go retains text/template instead of the unrelated main-branch html/template change.

Related

Control-D-Inc#229

[maghuro]

v1.x parity update:

• ported the final reviewed Prefer Merlin dnsmasq hooks and fix Guest Network Pro integration #2 internal/router/merlin/merlin.go and regression suite, including durable Merlin state-directory setup and fail-closed conf-file= / conf-dir= / conf-script= verification;
• internal/router/nvram/nvram.go already matched Prefer Merlin dnsmasq hooks and fix Guest Network Pro integration #2;
• the only intentional implementation difference from Prefer Merlin dnsmasq hooks and fix Guest Network Pro integration #2 is v1.x retaining text/template in the dnsmasq package;
• merged the latest [v1.x] Preserve existing Merlin dnsmasq.postconf content #6 base into this branch without changing the validated tree.

CI on the pre-base-alignment tree passed Ubuntu, macOS and Windows. The current merge-only head is being revalidated by CI.