fuzz: Add optimized AFL seeds and fix fuzz test bugs

Seeds added for disk/archive format parsers:
- ocaml-gpt: signature (8B), header_only (100B)
- ocaml-mbr: header_only (512B), zeroes (512B valid MBR)
- ocaml-cpio: header (110B), valid_archive (248B)
- ocaml-squashfs: magic (4B), superblock (96B)
- ocaml-tar: header (512B), valid_entry (1.5KB)

All seeds kept small (<1KB where possible) for efficient AFL mutation.

Fixes:
- cpio: Validate namesize > 0 before String.sub (crash on malformed input)
- squashfs: Fix compression test to check roundtrip validity