fix: handroll mpl facilities

[Becheler]

Continue removing MPL from the codebase to limit build times and dependencies.
But this time there is no direct C++ equivalent to the MPL features, so there is a small utility graphml_detail namespace.

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

What this PR does

The GraphML reader and writer kept a compile-time list of the value types they support (bool, int, double, string, and so on) and used three Boost.MPL tools on it:

• mpl::vector (the list of types),
• mpl::for_each (visit each type once, to try each candidate when parsing),
• mpl::find (get a type's position to look up its name).

Their replacement is ~15 lines of plain C++ in a small graphml_detail namespace:

• value_type_list type list,
• for_each_value_type that visits each type in order
• index_of_value_type that gives a type's position.

No behavior change, and no new dependency (no Boost.MP11).

Motivation

Part of the ongoing effort to shrink Boost.Graph's dependency footprint. Boost.MPL is an old, heavy, slow-to-compile library that modern C++ (<type_traits> plus small hand-written helpers) can replace directly. This removes the last direct MPL use from BGL that would not be breaking public API.

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]

Compiler-warning counts vs develop (auto-generated).
PR run 29508781546 vs develop run 29503421610 (ef7032f35d).

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

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 29508781359 vs develop run 29503421582 (ef7032f35d).

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

Dependency	develop	PR	Δ
mpl	30	29	-1

Transitive Boost modules: 69 → 69 (0)