No EntryPoint placement builds in every target: C# rejects it at file/namespace scope, C++/Rust lose main inside a namespace, and Rust/Go drop it silently inside a class #128

What's wrong

EntryPoint exists so that "the name, the parameter spelling and the wiring that runs it are the generator's business" (Coder/Ast/EntryPoint.cs). In practice each generator writes it exactly where it sits in the AST, and no placement produces a runnable program in every target:

C# writes a bare public static … Main in every position (CSharpGenerator.GenerateEntryPoint, Coder/Languages/CSharpGenerator.cs:926-943). EntryPointTests.Schema_TakesAnEntryPointAsAMemberButNotAsAStatement accepts it as a class member on purpose, "which is where C# puts Main", but that is exactly the position C++, C, Rust and Go can't use.

Repro (confirmed with dotnet build / g++ / rustc)

EntryPoint main = new() { ReturnsExitCode = true };
main.Body.Add(new ReturnStatement(new LiteralExpression<int>(0)));
SourceFile file = new("app");
NamespaceDeclaration ns = new("App");
ns.Members.Add(main);
file.Members.Add(ns);
// C#:   namespace App { public static int Main() { return 0; } }  -> CS0116
// C++:  namespace App { int main() { return 0; } }                -> ld: undefined reference to `main'
// Rust: pub mod App { ... fn main() {...} }                       -> error[E0601]

Why it matters

A caller who wants one AST to produce a runnable program in every language has no placement that works. Two of the failures (Rust and Go inside a class) produce no error or note at all: the program just has no entry point.

Suggested fix / acceptance criteria

• C#: an EntryPoint outside a type is wrapped in public static class Program { … }. Inside a type it stays as it is today.
• C++ and Rust: an EntryPoint inside a namespace or module is written at file scope, where the toolchain looks for it. For Rust, a root fn main can call App::run.
• C, C++, Rust and Go: an EntryPoint inside a class is written at file scope, or at least reported with WriteInexpressible, and never dropped silently.
• At least one placement (e.g. inside a NamespaceDeclaration) is compiled and linked by every language's compile test.