Find a better way to build libraries

Currently, building trunk libraries for compilers can take a day or longer.

It might be a better idea to apply a priority system based on compiler and library usage to spread out builds and not spend time on building things that nobody uses anyway.

Would require a registry of some sort where we store statistics on compilers and libraries.

Would perhaps require storing/loading data about when a library has been last been built.

Start by reviewing the existing process for building trunk compiler libraries and identify where build selection and timing are controlled. Clarify how compiler and library usage statistics, build timestamps, and a registry would fit that process. Done means a concrete, agreed design for prioritising builds and avoiding unused work.

Written by the indexing model from the issue text.