[v1.x] Discover dnsmasq lease files created after startup

[maghuro]

Important

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

Targeted at the maintained v1.x router-support line. This is the v1.x port of draft #5.

Summary

Make DHCP client discovery pick up dnsmasq lease files which appear after ctrld starts, including Asuswrt-Merlin 3006 Guest Network Pro / SDN instances.

• synchronize access to the shared lease-file registry;
• return EdgeOS's intended lease directory instead of discarding it;
• watch Merlin's /var/lib/misc lease directory;
• install the directory watcher before initial lease-file discovery, closing the startup blind window;
• scan the watched directory for already-existing dynamic dnsmasq-*.leases files;
• dynamically register newly created matching lease files;
• preserve continued handling across create/write/remove/recreate events;
• add regression coverage for Merlin SDN patterns and initial directory scanning.

Relation to Merlin SDN

The DNS hook work in #7 fixes forwarding for Guest Network Pro. This draft fixes the separate DHCP/client-discovery lifecycle so SDN leases can also appear after ctrld startup.