okfgen: re-measure parallel dotnet msbuild queries now that E13 removed the obj/ collisions #172

Extracted from ROADMAP.md (producer, "Measured and dropped: parallel dotnet msbuild queries").

Parallel queries were dropped on 2026-09-15. They were 37–40 % faster, but on a cold tree they lost projects to MSB3491 write collisions in a shared referenced project's obj/, and they did it silently: the run still exited 0.

E13 met the precondition:

• -p:BuildProjectReferences=false;
• IntermediateOutputPath redirected into producer-owned scratch.

A query now builds nothing and writes no file into the scanned repository's obj/.

Before revisiting

• Measure again. The old timings were for a query that built its whole reference closure, which is code that no longer exists.
• StageDeadline is not thread-safe. It is consulted per project inside the query loop.
• No one has shown that it is safe to run a repository's own MSBuild logic as several copies at once. A query evaluates that logic, and it can write wherever it likes.