Add Function and MutatingFunction type-erased callable wrappers

[Copilot]

DRAFT.md describes how protocol/protocol_view can implement the standard library's callable wrappers. This PR adds the concrete interface headers and tests that demonstrate this.

New interfaces

• interface_function.h — struct Function { int operator()(int) const; } — models std::copyable_function<int(int) const>
• interface_mutating_function.h — struct MutatingFunction { int operator()(int); } — models std::copyable_function<int(int)>

Standard library type	Protocol equivalent
std::copyable_function<int(int) const>	protocol<Function>
std::function_ref<int(int) const>	protocol_view<const Function>
std::copyable_function<int(int)>	protocol<MutatingFunction>
std::function_ref<int(int)>	protocol_view<MutatingFunction>

// Any const-callable fits protocol<Function> — lambdas, structs, etc.
xyz::protocol<xyz::Function> f(std::in_place_type<DoubleIt>);
f(3);  // → 6

// protocol_view<const Function> is a non-owning reference — like std::function_ref
DoubleIt d;
xyz::protocol_view<const xyz::Function> view(d);

// protocol<MutatingFunction> captures mutable state with deep-copy semantics
xyz::protocol<xyz::MutatingFunction> counter(std::in_place_type<Counter>);
counter(10);  // accumulated state; copy starts independent

Template bug fix

protocol.j2 and protocol_manual_vtable.j2 generated requires(const T& t) {} when an interface has no const methods — an empty requires-expression that GCC 13 rejects. The const concept now emits true in that case.