Virtual threads under load

1. static int vtFanout(int tasks) — run tasks virtual threads via newVirtualThreadPerTaskExecutor, each sleeping 20ms then adding 1 to an AtomicInteger; return the final count. Must work for 10,000 tasks (a platform pool of equal size would blow the 30s sandbox ceiling — VTs make it trivial). 2. static boolean isVirtualInside() — start a virtual thread that records Thread.currentThread().isVirtual() and return that record. 3. static int platformParallelism() — return Runtime.getRuntime().availableProcessors() (the honest CPU-bound ceiling). 4. static boolean virtualThreadsHelpCpuBound() — return false (VTs help waiting, not computing).

Difficulty: advanced