Deliver the multithreaded build performance win #15025

Performance is one of the most important reasons for the multithreaded MSBuild project. The goal of this epic is to make builds measurably faster in the majority of scenarios when running with -mt.

Steps

• #13766 #14236 Make PerfStar numbers reproducible so the baseline can be trusted [1.5 dev weeks]
• #14020 #14839 Close the multithreaded evaluation regression seen in pipelines [1 dev week]
• #13865 #13028 Remove lock and cache contention that limits thread scaling [2 dev weeks]
• #14113 Cut IPC overhead for tasks that run out of process [1 dev week]
• #14210 Clear the regression multithreading introduced into non-MT builds [0.5 dev weeks]

Stakeholders

Everyone building with -mt: .NET SDK CLI users, Visual Studio, and VMR and partner CI — whose build times are the reason multithreaded mode exists.

Definition of done

-mt is faster than the multi-process baseline on the majority of tracked PerfStar scenarios, cold and warm; non-MT builds show no regression from multithreading work; and PerfStar numbers reproduce in manual runs so the result is trustworthy.