feat(selfhost): function types (func(Params) -> Ret)

- Add tekFunc AST node and TypeExprList linked list for params
- Parse func(T, U) -> R syntax in parser.bux
- Resolve tekFunc to tyFunc in sema.bux
- Track original TypeExpr via Symbol.refType for params/lets/consts/funcs
- Build C function-pointer type names in hir_lower.bux (Ret (*)(Params))
- Lower indirect calls through function-typed values as hCallIndirect
- Add CBE_CParamDecl helper in c_backend.bux to embed name inside (*)
- Emit hCallIndirect and function-pointer variable declarations
- _test_funcptr now builds and runs with selfhost buxc2

src/types.bux

@@ -183,6 +183,7 @@ func Type_ToString(t: Type) -> String {
     if t.kind == tyNamed { return t.name; }
     if t.kind == tyTypeParam { return t.name; }
     if t.kind == tyPointer { return String_Concat("*", t.innerName1); }
+    if t.kind == tyFunc { return t.name; }
     return "?";
 }
 }