#2776 Expert arena load threads

Descripción

On Windows the arena is usually already in the file cache, so loading it is a RAM-to-RAM copy bound by the 6 copy threads.
`STRATA_ARENA_THREADS` (default 14 in the branch): 31.6 GiB at 6.5 GiB/s instead of 5.3 on a 7950X. Feel free to choose a
different default (e.g. physical cores - 2).

Full notes and measurements: https://github.com/apatassini/Strata/blob/qwen38-16gb/QWEN38.md

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