Append-Only Versioning

Old struct in your editor: struct cfg_v1 { int mode; int level; };. Implement size_t cfg_v1_size(void) returning sizeof(struct cfg_v1), and — defining struct cfg_v2 { int mode; int level; int flags; int reserved; }; — int cfg_v2_prefix_compatible(void) returning 1 iff the v2 members that v1 knew sit at identical offsets (verify with offsetof, do not hardcode 1).