map built 1024 per-leaf arrays and concatenated them on the joining thread,
a serial gap between back-to-back maps long enough for the workers to park.
At 20 threads spectral-norm took about 27 ms against Rayon's 18. tabulate
sizes the result once and each task stores its own run of elements; the
same workload now runs 10-14 ms against Rayon's 13-16, interleaved in one
session. tabulate's body is a parallel region entry, certified like a tasks
body. Needs the runtime in @geastack/compiler 1.0.36.