Fix fcontext stack size on 64KiB pages #73
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On aarch64 kernels with 64KiB pages (e.g. GH200 aarch64+64k), NLE hardcodes STACK_SIZE=64KiB. create_fcontext_stack() uses the first page as a guard page; with a 64KiB page size this leaves no usable stack space and results in a segfault on first reset()/context switch. This change computes an effective stack size at runtime and ensures at least 2 pages (guard + usable) via sysconf(_SC_PAGESIZE), while keeping existing behavior on 4KiB page systems. Tested on GH200: 2000 continuous resets test passed without a segfault.