fix(memory): use checked arithmetic in memfd_backed region sum

[dan1d]

Summary

The memfd_backed function uses an unchecked .sum() to calculate the total size of memory regions:

let size = regions.iter().map(|&(_, size)| size as u64).sum();

This could theoretically overflow if given sufficiently large region sizes.

This PR applies the same fix that was made to snapshot_file in commit 837c2e7: replace the unchecked sum with try_fold and checked_add, returning MemoryError::OffsetTooLarge on overflow.

Changes

• Replace .sum() with .try_fold() + checked_add() in memfd_backed
• Return MemoryError::OffsetTooLarge on overflow (consistent with snapshot_file)

Test plan

• Existing unit tests pass
• The fix mirrors the pattern already used in snapshot_file (lines 909-912)

Related

• Commit 837c2e7 fixed the same pattern in snapshot_file
• This ensures consistency across memory allocation functions