SiteScriptCompileCache.Hits is a process-global counter; other test classes in
the assembly compile scripts concurrently (not in this serialized collection),
so an exact post-Clear count is non-deterministic under a full-assembly parallel
run. Assert.Same(r1,r2) remains the definitive proof of one shared Roslyn
compile. Integration-surfaced; belongs with plan R2-03 T5/T6.