move padded layouts and dims from Experimental to the standard namespace

[nmm0]

Closes #404

[Copilot]

Pull request overview

Moves layout_left_padded / layout_right_padded and dims out of the experimental/proposed namespace into the standard namespace to align with their C++26 standardization (closes #404).

Changes:

• Update padded layout definitions and related traits/usages to live in MDSPAN_IMPL_STANDARD_NAMESPACE / Kokkos:: rather than ...::Experimental::.
• Update dims usage to MDSPAN_IMPL_STANDARD_NAMESPACE::dims and adjust the experimental header accordingly.
• Refactor tests, compilation tests, and benchmarks to use the new namespace locations.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_submdspan_static_slice.cpp	Updates expected padded layout types to Kokkos::layout_*_padded.
tests/test_submdspan.cpp	Switches padded layout aliases to Kokkos::layout_*_padded; minor whitespace cleanup.
tests/test_layout_padded_right.cpp	Updates padded layout and detail trait references from experimental/proposed to standard namespace.
tests/test_layout_padded_left.cpp	Updates padded layout and detail trait references from experimental/proposed to standard namespace.
tests/test_dims.cpp	Updates dims usage to MDSPAN_IMPL_STANDARD_NAMESPACE::dims.
include/experimental/__p2642_bits/layout_padded_fwd.hpp	Removes proposed-namespace wrapping so padded layouts are declared in the standard namespace.
include/experimental/__p2642_bits/layout_padded.hpp	Removes proposed-namespace wrapping and updates internal references accordingly.
include/experimental/__p2630_bits/submdspan_mapping.hpp	Updates submdspan mapping code to refer to layout_*_padded in the standard namespace.
include/experimental/__p2389_bits/dims.hpp	Moves dims alias to the standard namespace (and removes proposed namespace wrapper).
include/experimental/__p0009_bits/layout_stride.hpp	Updates trait checks for padded mappings to reference standard detail::is_layout_*_padded_mapping.
include/experimental/__p0009_bits/layout_right.hpp	Updates converting ctor constraints/checks to use standard padded-mapping traits and checks.
include/experimental/__p0009_bits/layout_left.hpp	Updates converting ctor constraints/checks to use standard padded-mapping traits and checks.
compilation_tests/ctest_layout_convertible.cpp	Updates compilation-time checks to use Kokkos::layout_*_padded.
benchmarks/sum/sum_3d_common.hpp	Removes unused proposed-namespace alias (now that padded/dims moved).
benchmarks/fill.hpp	Removes unused proposed-namespace alias (now that padded/dims moved).

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[mhoemmen]

Should we consider also regenerating the single-header file for Compiler Explorer as part of this PR? It would be excellent for the single-header version to have the same namespaces as the installable version. Thanks! : - )