Adds docs/nesting-engines.md: which engine suits which jobs, the old plug-in
names the registry maps, retired engines, and the rule that an engine change
lands only when it beats that engine's current benchmark result with every
layout valid. README, release and automatic-nesting docs drop the bundled
plug-in wording; AGENTS.md now places built-in engines in
OpenNest.Engine/NestingEngines and keeps only external plug-ins out of the
solution.
