C#: use LayoutKind.Sequential for single-field exposed structs (IL2CPP wasm ABI)

[Grantim]

Problem

Unity WebGL (IL2CPP) builds of MeshLib's wasm bindings link with 622 wasm-ld: warning: function signature mismatch, one per C API function returning an MR::*Id; those calls trap at runtime. The ~1000 functions that take an Id by value get no warning (same i32 on both sides) but silently receive a pointer to a stack copy instead of the value.

Root cause: exposed structs are emitted with LayoutKind.Explicit + FieldOffset. IL2CPP compiles explicit-layout structs to a union with padding (union { struct { ... }; uint8_t padding[N]; }), and Clang's wasm32 ABI only flattens single-element structs to scalars. So a one-field struct becomes an aggregate on the IL2CPP side (returned through a hidden pointer, passed by pointer) while the C side uses a plain int. IL2CPP compiles sequential structs to plain C structs (e.g. struct Int32_t { int32_t ___m_value; };), which Clang flattens exactly like the C side. Multi-field structs are passed indirectly on both sides and are unaffected.

Fix

Exposed structs with exactly one non-static field are emitted with LayoutKind.Sequential and without FieldOffset attributes. For one field both layouts are trivially identical (offset 0, size and alignment of the field), so nothing else changes: no DllImport signatures, no C API. All other exposed structs keep LayoutKind.Explicit.

Verification

• Regenerated all test outputs (parser built with gcc 14 + the llvm-22.1.8 libs, like CI). The diff is confined to the attribute lines of the single-field structs (ConvCtorExposed, ExposedLayoutB, ExposedLayoutC, ConstNonconstConflicts, NameConflictsExposed::A, DeclOrder::*, and the std::array wrappers) plus the new test function.
• dotnet build of the three generated C# test projects: 0 warnings.
• The ABI difference was reproduced with Unity's own emcc: IL2CPP's union shape gives (i32 sret) -> void and pass-by-pointer; the plain one-field struct gives () -> i32 and pass-by-value, matching MeshLib's C API.
• Added test_exposed_c() covering a by-value parameter and return value of a single-field exposed struct.

Supersedes #48, which fixed the same bug at the DllImport level with more code.

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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