DDN_DF_MAX_THREADS when set and valid (at least 1, clamped
to the core count), otherwise the core count.
dfThreads
fn
size_t dfThreads() @trustedThe effective ddn.df thread count.
Returns
Throws
DataFrameException when DDN_DF_MAX_THREADS is set but is
not a positive integer - a misconfiguration must surface where the pool is first needed, not silently halve the machine.