[ARM/ARM64] C# interop broken due to HVA ABI

[JunaMeinhold]

On ARM/ARM64, the current Hexa.NET.ImGui bindings break because C# cannot handle homogeneous vector aggregates (HVAs) correctly. Functions that take or return ImVec2, ImVec4, or similar types fail at runtime due to ABI mismatches.

Root Cause:

• ARM64 ABI passes HVAs in SIMD registers.
• C# P/Invoke does not support HVAs, leading to corrupted arguments/returns.
• Works on x86/x64 because ABI rules differ.

Proposed Solution:
Develop a C++ → C generator for the bindings:

1. Automatically generate C ABI-safe wrapper functions from the ImGui C++ API.
2. Replace vector-type parameters/returns with scalar equivalents or plain structs with const ptr.
3. Expose only C-style functions to C#, ensuring ABI compatibility across ARM/ARM64 and x86/x64.

Benefits:

• Eliminates manual binding errors.
• Ensures ARM64 compatibility.
• Keeps API surface consistent for managed consumers.

Action Items:

• Prototype CPP→C generator for a subset of ImGui functions. HexaGen#29
• Replace existing vector-type interop in Hexa.NET.ImGui.
• Validate ARM64 runtime behavior with generated wrappers.

Notes:

• This approach is preferable to attempting direct P/Invoke of C++ vectors.
• Could be extended in the future for other C++ libraries with similar issues.