Migrate public traits from MPL to STL (breaking change)

[Becheler]

Reduce the number of MPL dependencies. Probably not relevant for 1.92 given how late we are in the cycle.

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• [] Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)

• No

• Patterns that break (all need the same two things at once: an overload set keyed on the exact mpl:: type, fed a BGL trait's ::type, which is the type that changed):

  • mpl::true_/mpl::false_ overloads driven by a BGL trait's ::type. The tag no longer matches the overload:
    • Ex: f(typename is_directed_graph<G>::type()) against void f(mpl::true_) / void f(mpl::false_).
  • Exact-type checks
    • Ex: static_assert(is_same<is_directed_graph<G>::type, mpl::true_>::value).
  • Template specializations keyed on mpl::true_/mpl::false_ that are instantiated with a BGL trait's ::type.

• Patterns that still work:

  • Anything reading ::value (overwhelming majority ?)
    • Ex: if (is_directed_graph<G>::value)
    • Ex: if constexpr (is_directed_graph<G>::value)
  • Feeding the trait into mpl::if_ / mpl::and_ / mpl::not_ and similar. Those read ::value, so std::integral_constant works.
  • Overloads that take a generic template<class B> parameter and read B::value instead of matching a concrete tag type.
  • Overloads already written on std::true_type / std::false_type. Before this change they would have had to convert first.

What this PR does

Converts Boost.Graph's public traits from Boost.MPL to standard <type_traits>. This is a breaking change: the traits now derive from std::integral_constant / std::true_type / std::false_type instead of boost::mpl::bool_ / true_ / false_.

• graph_selectors.hpp: directedS/undirectedS/bidirectionalS's is_directed_t and is_bidir_t now std::true_type/std::false_type.
• graph_traits.hpp: is_directed_graph, is_undirected_graph, is_multigraph, is_incidence/bidirectional/vertex_list/edge_list_graph, is_adjacency_matrix, is_directed_uni/bidirectional_graph, has_*_property, has_bundled_*, and is_no_bundle now derive from std::integral_constant (file MPL-free)
• graph_mutability_traits.hpp: the whole graph_has_* and is_mutable_* family now derive from std::integral_constant (file MPL-free)
• reverse_graph.hpp: is_reverse_graph now std::true_type/std::false_type; internal mpl::if_ to std::conditional (file MPL-free)
• labeled_graph_traits.hpp: converted the labeled mutability traits (graph_has_*_by_label, is_labeled_mutable_*) and determine_mutability from mpl::bool_/and_/if_ to std::integral_constant/std::conditional, (file MPL-free)
• pending/property.hpp: has_property and is_no_property now on std; internal mpl::if_ to std::conditional.
• test/: test_direction, test_properties, test_construction, test_destruction update their tag-dispatch overloads from boost::mpl::true_/false_ to std::true_type/std::false_type to match the new trait types.
• adjacency_list.hpp: converted is_random_access and is_distributed_selector from mpl::true_/false_ to std::true_type/false_type making public adjacency_list_traits::is_rand_access a std type.

Motivation

Reduce the number of MPL dependencies.

Testing

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 29566287405 vs develop run 29503421582 (c9ce31788e).

Header-inclusion weights (graph files pulling each direct dependency in):

Dependency	develop	PR	Δ
mpl	30	26	-4

Transitive Boost modules: 69 → 69 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 29566287403 vs develop run 29503421610 (c9ce31788e).

Job	Baseline	After	Delta
macos (clang, 14)	4	4	0
macos (clang, 17)	4	4	0
macos (clang, 20)	4	4	0
ubuntu (clang-19, 14)	4	4	0
ubuntu (clang-19, 17)	4	4	0
ubuntu (clang-19, 20)	4	4	0
ubuntu (clang-19, 23)	4	4	0
ubuntu (gcc-14, 14)	10	10	0
ubuntu (gcc-14, 17)	10	10	0
ubuntu (gcc-14, 20)	10	10	0
ubuntu (gcc-14, 23)	10	10	0
windows_msvc_14_3 (msvc-14.3)	971	971	0