Friend operator (IsFriend, two operands) generates a three-operand operator in C# (CS1534) and Rust (E0050) #231

What's wrong

FunctionDeclaration.IsFriend is documented as "declared inside a type but is not a member of it ... how a symmetric operator is written beside the type" (Coder/Ast/FunctionDeclaration.cs:~146-155). A friend operator therefore declares both of its operands itself. Two generators ignore that:

• C#: CSharpGenerator.TakesImplicitOperand (Coder/Languages/CSharpGenerator.cs:1072-1073) was added by 874626e (C#: member operators lose their implicit operand, so binary + silently becomes unary + and unary/conversion operators fail with CS1019 #126). It prepends Point self to every operator or conversion that has an enclosing type, without checking IsFriend. Before that commit this case generated a correct two-parameter operator, so this is a regression.
• Rust: the operator-trait writer (Coder/Languages/RustGenerator.cs, ~:841-855) always writes self followed by every declared parameter.

Reproduction

A struct Point { int x } with an operator +, Kind = Operator, IsFriend = true, parameters a: Point, b: Point, returning a:

// C++ (correct)
friend Point operator+(Point a, Point b)
// C#
public static Point operator +(Point self, Point a, Point b)   // error CS1534: Overloaded binary operator '+' takes two parameters
// Rust
impl std::ops::Add for Point { type Output = Point; fn add(self, a: Point, b: Point) -> Point {...} }   // E0050

Both errors were confirmed with dotnet build and rustc. C and Go also gain a spurious receiver (Point_add(Point* self, Point a, Point b) and func (self *Point) Add(a, b)). Those compile, but don't match the declared arity.

Suggested fix

• C#: return false from TakesImplicitOperand when function.IsFriend. The operator is already static.
• Rust: when IsFriend, bind the first declared parameter as self (fn add(self, rhs: Point), rebinding the first parameter's name in the body), or write the operator out as unsupported.
• C and Go: skip the receiver for friends, as WriteReceiver already does for IsStatic.
• Add a friend + to CompiledExemplar and compile it in the C# and Rust compile tests.