Add std::vector constructor overloads and tests

[Ayuse]

Add C++ std::vector<int> constructors in Rust using the overload! macro.

• Empty constructor: Creates a vector with no elements.
• Count constructor: Creates a given number of elements, each set to zero.
• Count-and-value constructor: Creates a given number of copies of a value.
• Slice constructor: Copies elements from a Rust slice into a new C++ vector.

1. Write Rust wrappers for that C++ type, for 3-5 different overloaded method names:
   • Create a new file named after your type in overloading-macros/cpp-overload-test/src/stdcpp
   • Wrap constructors first, writing tests to make sure the correct constructor is called
   • Explain any potential compatibility issues in a Limitations doc section
   • Explain usability issues in an Ergonomics doc section
   • Check that you have implemented all the overloads for each method name, or documented why you can't
   • Push your changes and wait for a review
2. Add a separate example with some failing overloads
   • Create a new file named after your type in overloading-macros/cpp-overload-test/examples
   • Wrap accessor/modifier methods
   • Explain how they fail in an Incompatibilities doc section
   • Explain how the overload failures could be resolved in a Workarounds doc section
   • Optional: If there's a C++ feature with no equivalent in Rust, explain that in an Inexpressible Overloads doc section
   • Push your changes and wait for a review
3. Update the overload macro compatibility table Markdown doc
   • If you haven't checked some kinds of overloads, put a ？in those squares
   • Push your changes and wait for a review

AI disclosure: Codex helped to explain and understand the problem, then gave conceptual solution.

[teor2345]

It looks like you're missing some overload sets (we want 3-5 different function names), and some overloads within those sets (we want all of the possible overloads).

Please focus on completing this PR so you make it through all 3 steps before the application period ends.

[teor2345]

It would be clearer to use a Rust type alias T and C++ typedef item_t. This also makes it easier to change the type to see if overloads conflict with other types.

See the forward_list example for how this can be done.

[teor2345]

Please research how C++ shows allocation errors, and what happens at the FFI boundary to those errors, and document it here.

[teor2345]

Rust requires a PhamtomData<T> in this struct, to show that it contains instances of type T behind the pointer. This makes sure that auto-traits are implemented correctly.

Suggested change

	pub struct StdVector(*mut c_void);
	pub struct StdVector(*mut c_void, PhantomData<T>);

[teor2345]

We want to eventually make this code generic. Would it work for Rust zero-sized types (ZSTs)?

Please research how they work in Rust and C++, and document the limitations of this implementation. If you want you can instead add an implementation and a test with ZSTs, but that could be a lot trickier.

[teor2345]

Please add as many of these constructors as is reasonable, or document why they can't be added. We need to implement the full overload set to show they are all compatible with each other.

See the forward_list example for details.