fix(mm): saturate offset/page-round arithmetic in mremap/mincore/vma helpers#127
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…helpers Nine unchecked additions on user-derived length/offset values: page round-ups ((x + PAGE_SIZE - 1)) in mincore_ops/mremap_ops, mapping end+extra and new_addr+new_size in mremap_ops, file-offset continuity / split-offset updates in mmap_region, and pgoff+nr_pages in vma_merge. A value near usize::MAX overflows -> ring-0 panic under overflow-checks. Use saturating_add throughout: for the contiguity predicates a saturated end simply reads as non-contiguous (fail-safe), and the split/round paths can no longer panic. All are exported mm helpers not yet reached from a live mmap-family syscall (latent); the wired mmap/mprotect/slab/page_alloc /vmalloc/page_table paths were verified clean.
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Nine unchecked user-derived additions (page round-ups, mapping end+extra, new_addr+new_size, file-offset continuity/split updates, pgoff+nr_pages) → saturating_add. Latent (unwired mm helpers; wired mmap/mprotect/slab/page_alloc/vmalloc/page_table verified clean by a Codex mm pass). Gate: clippy -p oncrix-mm ✓ + build ✓ + fmt ✓.