Bound Asuswrt-Merlin PreRun waits

[maghuro]

Important

Local staging/review draft in the fork only. Per upstream contribution guidelines, an upstream issue must be filed before any PR is submitted to Control-D-Inc/ctrld.

Summary

This draft prevents the Asuswrt-Merlin startup path from waiting forever when required paths or dnsmasq never become available.

Changes

• replace the unbounded waitDirExists loop with a timeout-aware helper;
• bound waits for /tmp and /proc to 60 seconds;
• bound the dnsmasq pidof wait to 60 seconds;
• treat normal pidof "not found yet" exit status as retryable, while propagating real execution errors;
• return contextual startup errors instead of wedging ctrld indefinitely;
• add unit tests for success, probe failure and timeout behavior.

Scope

This is intentionally separate from dnsmasq/SDN/NVRAM integration work (#1/#2) and Merlin service wrapper work (#3).