C#: pass single-field exposed structs through DllImport as their scalar (IL2CPP wasm ABI) - #48

Problem

Unity WebGL (IL2CPP) builds of MeshLib's wasm bindings link with 622 wasm-ld: warning: function signature mismatch, one for every C API function returning an MR::*Id:

wasm-ld: warning: function signature mismatch: MR_EdgeId_undirected
>>> defined as (i32, i32) -> void   in GameAssembly.a(<IL2CPP object>)      <- IL2CPP call site
>>> defined as (i32) -> i32         in libMeshLibC2.a(unity_0_cxx.cxx.o)    <- MeshLib C API

Those calls trap at runtime. Worse, the ~1000 C API functions that take an Id by value get no warning at all (same i32 on both sides), but IL2CPP passes a pointer to a stack copy where the C function reads the id.

Root cause: IL2CPP emits every C# struct as struct { union { struct { ... }; uint8_t padding[N]; }; }, and Clang's wasm32 ABI only flattens single-element structs. So a one-field exposed struct becomes an aggregate on the IL2CPP side (sret return, pass by pointer), while the C side returns/accepts a plain int. Multi-field structs (Vector3f, Box3f, ...) are unaffected: both sides already pass them indirectly. Reproduced with Unity's own emcc, and the 622 warned functions are exactly the set of P/Invokes returning an Id type.

Fix

If an exposed struct has exactly one non-static field, and that field is an arithmetic type other than bool or an enum, the C# generator now declares the DllImport parameters/return as that scalar and converts at the boundary (v.x on the way in, new T {x = ...} on the way out). A scalar has the same ABI on every runtime and target. The public C# API and the C API are unchanged (in C both spellings have the same ABI on all supported platforms). Pointer paths (this, in, defaulted params) are untouched.

The two constructor emission paths (this = ... and the Box wrapper's *(T *)_UnderlyingPtr = ...) now go through the return binding so they compose with this; for other types they produce the same text as before.

Verification

• Regenerated all test outputs with the parser (gcc 14 + the llvm-22.1.8 libs, like CI). The diff is confined to single-field exposed structs (ConvCtorExposed, ExposedLayoutB, ExposedLayoutC, DeclOrder::*), including constructors and the std::function<ExposedLayoutC(ExposedLayoutC)> callback wrapper.
• dotnet build of the three generated C# test projects: 0 warnings.
• Added test_exposed_c() to cover a by-value parameter and return value in a plain function.

Follow-up in MeshLib: bump mrbind and regenerate the C# bindings; the Unity wasm link should then be free of function signature mismatch warnings.

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