Layout fingerprint of the structs that cross DLL boundaries.
A package DLL builds its property tables as static arrays and hands the editor a pointer and a count. The editor walks them with its own compiled sizeof, so both sides must agree on the layout exactly. If they do not, element 0 reads fine and every later element is read from the middle of a struct; a const char* read that way is non-null garbage that passes null checks and crashes far away.
The append-only rule of the metadata structs protects a single struct read through a pointer, not an array, whose stride changes. Hence this handshake: every package DLL exports its compiled values (ReflectionAbi.cpp, linked into each package by BuildFileGenerator) and the loader refuses a package that disagrees, naming it and saying to rebuild.
It catches size changes, not same-size reorderings: for those, bump kDekiReflectionAbiVersion by hand.
1.18.0