Voxels.bIsInside returns true for every point: bool return values in Interop.cs need MarshalAs(UnmanagedType.I1)

[dl-eigenart]

Voxels.bIsInside (new in PicoGK 2, discussion #95) returns true for every point I give it, inside or outside, at every voxel size I tried (20, 5, 2, 1 and 0.5 mm), before and after boolean operations. The signed distance values in the field are fine at the same points, so the runtime knows where the surface is. The problem is in the C# declaration.

Voxels_bIsInside returns a C++ bool, one byte in AL. The declaration in Internals/Interop.cs (line 415) has no MarshalAs, so .NET reads the return as a four byte Win32 BOOL and treats anything non-zero as true. I called the same native function through three private declarations to see what actually comes back:

x [mm]	expected	Voxels.bIsInside	with [return: MarshalAs(UnmanagedType.I1)]	raw EAX
0	inside	true	true	0xC4000E01
45	inside	true	true	0xC4000E01
50.5	outside	true	false	0xC4000E00
200	outside	true	false	0xC4000E00

Sphere R = 50 mm, 5 mm voxels. The low byte is correct, the upper three bytes are whatever was left in the register. With I1 marshalling the function is right at every point. ScalarField_bGetValue works today only because the compiler happens to clear the full register there (raw EAX is a clean 0x00000000 / 0x00000001).

Interop.cs has 26 extern declarations returning bool and 7 bool parameters in extern signatures, none of them annotated. Which of them misbehave depends on the compiler and the platform, so I would fix all of them rather than the one I hit. A pull request with [return: MarshalAs(UnmanagedType.I1)] on the 26 returns and [MarshalAs(UnmanagedType.I1)] on the 7 parameters is here: #115. It changes 33 lines and nothing in the runtime.

One caveat: I am on Linux (Ubuntu 25.10, runtime 26.2.0 built from main at 0f26321 with the Dockerfile from #77, PicoGK at 0e6cf6b). I know Linux is not a supported platform. But the missing annotation is the same on macOS and Windows; it just has not bitten anyone there yet, as far as I can tell.

Repro is a single file, dotnet run -- innen2 5: https://github.com/dl-eigenart/picogk-repro

Would you prefer this as an issue on the PicoGK repo instead of a discussion?