Proposal: Add IReserveableVector to WinRT

This proposal introduces a new WinRT interface, IReserveableVector, that extends IVector<T> with a capacity-reservation method. It allows callers to preallocate storage before bulk insertion, reducing repeated reallocations and improving performance for append-heavy scenarios.

Motivation
IVector<T> exposes size and element access but no way to reserve capacity. When an app appends many elements, the underlying implementation may grow and copy repeatedly. A Reserve method gives callers explicit control over allocation, similar to std::vector::reserve or .NET List<T>.Capacity.

std::vector::reserve can make push_back more than twice as fast, and it can also make IVector::Append more than twice as fast. This usage has existed in C++ for thirty years, but WinRT lacks it. C++/WinRT and C#/WinRT should support implementing this interface when implementing IVector.

Proposed Interface

interface IReserveableVector
{
    void Reserve(UInt32 capacity);
}

Behavior

• Reserve(capacity) ensures the vector can hold at least capacity elements without reallocation.
• If capacity is less than or equal to the current capacity, the call is a no-op.
• If capacity is smaller than Size, it does not shrink the vector; existing elements are preserved.
• The method does not change Size or insert elements.

Start by locating the existing WinRT IVector interface definition and its C++/WinRT and C#/WinRT projection or implementation entry points. Review how interface additions are specified and tested, then determine the required Reserve behavior and confirm that size and existing elements remain unchanged.