CVE-2025-57807: ImageMagick BlobStream Forward-Seek Under-Allocation

For memory-backed blobs (BlobStream), SeekBlob() permits advancing the stream offset beyond the current end without increasing capacity. The subsequent WriteBlob() then expands by quantum + length (amortized) instead of offset + length, and copies to data + offset. When offset ≫ extent, the copy targets memory beyond the allocation, producing a deterministic heap write on 64-bit builds. No 2⁶⁴ arithmetic wrap, external delegates, or policy settings are required.