Why types and checks both exist

Every id, mode and kind is a struct around one integer, and every boundary that reads one still checks its value. The type stops a transposition at compile time. The check stops nonsense at run time. They are different jobs.

The types

A Mesh takes a geometry id, a material id and a node id in a row. As plain integers, any two could be swapped and the program would render nonsense. As GeometryId, MaterialId and NodeId, a swap does not compile. tests/compile_fail/ holds one file per type that proves it.

The same discipline covers Side, Blending, MaterialKind, LightKind, ShadeMode, Wrap, Filter and ColorSpace. It covers the units too: rotation_z(90.0) does not compile, because the compiler cannot tell degrees from radians.

What a type does not do

A struct's fields are open in Mojo. Blending(7) constructs, and so does changed.value = 7 after the fact. For a while the runtime checks had been dropped on the strength of the types. The two rasterizers could then read a value neither knew in opposite directions. The CPU asked "is it BLEND?" and the GPU asked "is it OPAQUE?", so a 7 was opaque on one and blended on the other.

The checks

Every enum-like type has is_valid, and every boundary asks:

Boundary	Checks
Material constructor	side, blending, kind
Texture constructor and validate	wrap, filter, color_space
Lighting resolution	The light's kind
Renderer.set_shading	The mode
check_triangle_state, on both backends	The blend policy and the texture id
rasterize_all and GpuRenderer.draw	Every triangle, before any work
flatten_textures	Every texture again, at upload

The kernel's remaining fallthrough branches ask the host's question, so a value that slips past everything falls the same way on both sides.

Before any band starts

rasterize_all validates every triangle before it hands work to a thread. A band skips triangles outside its rows, so without that step an off-screen malformed triangle raised on one worker and passed on four. The answer must not depend on visibility or on the worker count.