Fix layout stride converting explicit clause

[nmm0]

We forgot to add the padded layouts to the layout_stride converting constructor, see https://eel.is/c++draft/mdspan.layout#stride.cons-9

This PR adds tests for all the strided layout conversions/constructions/assignments from other layouts

[mhoemmen]

Thanks for the fix!

[Copilot]

Pull request overview

This pull request fixes the layout_stride converting constructor to properly handle conversions from padded layouts (layout_left_padded and layout_right_padded). The fix ensures that conversions from padded layouts to layout_stride are implicit (when extents are convertible), consistent with how conversions from layout_left, layout_right, and layout_stride itself are handled.

Changes:

• Added include for layout_padded_fwd.hpp to access padded layout type traits
• Extended the MDSPAN_CONDITIONAL_EXPLICIT clause to include padded layout mappings in the list of layouts that should have implicit conversion
• Added compile-time tests to verify constructibility, convertibility, and assignability for conversions between layout_stride and padded layouts

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
include/experimental/__p0009_bits/layout_stride.hpp	Added include for padded layout traits and extended the converting constructor's explicit clause to handle padded layouts
compilation_tests/ctest_layout_convertible.cpp	Added compile-time tests (guarded with C++17 check) to verify bidirectional conversion support between layout_stride and padded layouts

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