Cloud Hypervisor aborts on a VHDX header at the sequence max

c260341ca92f (Wei Liu, 2026-08-26). block/src/formats/vhdx/header.rs. Avoid header sequence number overflow. A plain add on the current header sequence aborted the VMM with “attempt to add with overflow” when the image already carried the maximum 64-bit value. The increment now returns an error instead of taking the process down. Found by fuzzing. Assisted-by Pi:Opus-5. CH HEAD is now this commit (v53.0). This is not 207985b8 (VHDX flush).