gc-stacks: Disable transparent huge pages for task stacks
With THP enabled=always, the first touch of an 8 MiB task stack faults in
a whole 2 MiB page, so every started task costs ~2 MiB RSS and the
zeroing dominates spawn-heavy code (fib(24) with @spawn at -t 1:
553 ms -> 133 ms).
Assisted-by: Claude Code (Opus 5.5)
