Document opt-in local workers and the per-process runtime ownership model.
Record the two parallel-only failures found and fixed in test
infrastructure. Record the measured results on the final code: serial
oracle 496.3s, -n 2 267.6s (1.85x), and two green -n 4 runs at 155.6s mean
(3.19x), all with identical skip and xfail sets and no leaked processes,
listeners, state, or runtime roots. Recommend -n 4 locally and explain
why -n auto was not run. The full serial run remains the release oracle.
