The arena constructor was Windows-only, so on Linux Available() was false: the guest heap fell back to the host allocator and direct memory landed wherever the kernel pleased. Titles that require a low virtual address rejected the mapping and re-issued the identical request, which surfaced as a start-up assert inside their own allocator.
Seed the window from the mappings that already exist instead of reserving it up front, because a PROT_NONE reservation of the whole range starves the kernel's own top-down mmap, which picks addresses just below the stack inside that range. Commit, reset and set-protection become portable so the arena owns its used ranges on both platforms, and fixed mappings are checked against the arena's used ranges rather than against EEXIST from a reservation.
The window-placement and no-overwrite checks in guest_memory were compiled only for Windows; they now run on Linux too, and the window check fails without this change.
Fixes#692
e4e0702946