[FEA]: P3242R4 Copy and fill for `mdspan`

This issue can be closed by implementing [P3242R4](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p3242r4.html)

Start by reading the linked P3242R4 paper to understand the proposed copy-and-fill operations for mdspan. The issue names no implementation files, tests, or entry points, so locate the existing mdspan support before deciding the scope. Done means implementing the paper's requested functionality and adding the corresponding project coverage.

Written by the indexing model from the issue text.