vm_page_alloc_contig silently ignores alignment, boundary, and low address constraints

Summary

vm_page_alloc_contig accepts alignment/boundary/low params but never enforces them: alist_alloc(alist,start,count) at :2809 takes no alignment param; only high is checked at :2819; low is only used in kprintf at :2841. Non-power-of-2 sizes and boundary constraints entirely unenforced. Caller proceeds assuming constraints met -> silent DMA misalignment or hardware malfunction. Fallback scanning allocator vm_contig_pg_alloc DOES enforce all constraints but vm_page_alloc_contig is tried first and trusted.