Fix lock contention in ProjectRootElementCache

`Microsoft.Build.Evaluation.ProjectRootElementCache` is protected by a single `_locker` that is acquired by every read and every write. All evaluator threads in a single MSBuild process converge on this lock for every resolution, capping in-process evaluation parallelism regardless of available cores.

Symptoms: Profiling shows ~1,600+ contention events on _locker

`WeakValueDictionary` is not thread-safe, so every lookup requires the lock. `BoostEntryInStrongCache` is O(n) and runs under the lock on every cache hit.