Fix invalid IL for generative methods declared with System.Void return type

[Thorium]

Problem

The unit-return fix from #518 emits I_ldnull before I_ret whenever the method's return type is unit-like:

let retUnit = retType = ILType.Void || retType.QualifiedName = (transType (convTypeToTgt typeof<unit>)).QualifiedName
...
if retUnit then ilg.Emit(I_ldnull)

The check also matches ILType.Void, so a ProvidedMethod declared with returnType = typeof<System.Void> (which transType maps to ILType.Void) gets a stray null pushed onto the stack of a void-returning method. That is invalid IL — the stack must be empty at ret in a void method — and the JIT throws InvalidProgramException the first time the method is invoked:

System.InvalidProgramException : Common Language Runtime detected an invalid program.
   at VoidReturn.Provided.Container.Widget.DoNothing()

(The equivalent lambda path in CodeGenerator.emitLambda already gets this right: it emits I_ldnull only for the FSharp.Core Unit return type, not for void.)

Fix

Split the check: retVoid keeps the empty-stack behavior with no ldnull; only a genuine FSharp.Core Unit return pushes the boxed unit value null.

Testing

• New generative regression test declaring a System.Void-returning method and invoking it from the loaded assembly — fails with InvalidProgramException before the fix, passes after.
• Full suite green: 161/161 (including the try-finally unit-return test added in Fix for issue #517 #518).

🤖 Generated with Claude Code

[Thorium]

This is a low-priority issue compared to 520 and 522, which are solving real production issues.