#488 pinned: honor STRATA_NO_LARGEPAGES on Linux; name the hugetlb pool shortfall

Descripción

Two fixes to the Linux large-page path in `reserve()` (`src/core/pinned.cu`):

1. **`STRATA_NO_LARGEPAGES=1` was Windows-only.** It is documented as a same-run A/B switch for the large-page path, but only the Windows branch read it; on Linux the `MAP_HUGETLB` path ignored it, so the switch silently did nothing. Honor it there too.
2. **The failure message hid the real cause.** It said "no hugetlb pool configured?" even when a pool exists but is smaller than the mapping. `MAP_HUGETLB` is all-or-nothing, so a 34 GiB pool cannot serve a 40 GiB arena and the whole mapping falls back to 4 KiB. Name the shortfall: the 2 MiB page count needed vs `vm.nr_hugepages`. (The Windows branch already names its byte count and error codes.)

Measured on a 39.97 GiB arena with a 34 GiB pool: it fell back to 4 KiB on every start until the pool was raised to 44 GiB. Details in `bench/results/2026-10-02-rtx-a3000-i7-12850hx/`.