The workspace forbids third party crates, and a thread pool is not needed:
every loop in this workspace that benefits from several cores iterates
independent items, which scoped threads express directly. Borrowing the
data removes the need for an Arc, and joining inside the scope means no
result escapes while a thread still writes it.
for_each_index and map_index split a range into one chunk per thread. Both
take a closure that is only called, never mutated, so the caller shares
input and produces output rather than racing on it. threads_for keeps a
short loop on one thread, because starting a thread to do a few
microseconds of work costs more than the work.
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