Structs a mod builds and structs a mod receives

1. A type a mod builds (definitions, options, screens, declarations): public fields and Default. Write a literal with ..Default::default() at the end, so a field added later never breaks the mod:

   rustWrap linesCopy

   let screen = Screen { width: 176.0, height: 166.0, ..Screen::default() };

   Where a field has no neutral value (an identity, such as a registry id), the type has a constructor that takes the required fields and leaves the rest at their defaults, and no Default that would invent one.

2. A type a mod only receives (event payloads, snapshots, contexts such as HurtSource, Victim or Hit): #[non_exhaustive]. A mod reads its fields but can't build it or match it without ... The game builds it through its crate's private built mirror and From.

The two can't overlap: #[non_exhaustive] forbids struct literals outside the crate.

The check

tools/mod_api_struct_kinds.py reads the sources and lists the structs that are neither: a mod can build them with a literal (named fields, all pub, not #[non_exhaustive]), but they have no Default and no constructor. python tools/mod_api_struct_kinds.py --summary prints the counts.

tests/test_mod_api_struct_kinds.py checks a downward-only ceiling, not equality. Parallel classification batches need not edit the ceiling; a decrease is accepted without a shared-line change. The ceiling is a regression budget, not proof that every newly added struct has a kind.

This source scanner cannot expand macro_rules! or build scripts, can count fixed-name declarations inside unused macro templates, and does not follow pub use reachability. Mirrors are excluded only at the crate's private built-mirror paths.

The batches go by module. Kind 2 is applied mechanically; which fields of a kind 1 type are required is decided by hand. Each batch updates the samples and this book in the same PR, with no aliases or shims.