Faster default starts
In matched 20-run public-API measurements, the default 2 vCPU / 4 GiB memory / 4 GiB disk sandbox reached running in 207 ms median (620 ms p95), and took 351 ms median from create through the first Python result (815 ms p95). Both runs had zero errors. See speed for the before figures and scope; Python execution itself did not get faster.