refactor: migrate mas symbols to boost::graph namespace

[Becheler]

#286
#297

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

• Moved mas_visitor, make_mas_visitor, default_mas_visitor, MASVisitorConcept into boost::graph
• Left deprecated boost:: aliases (Boost.URL macro pattern: MSVC-safe)
• Repointed internal uses: stoer_wagner_min_cut.hpp, mas_dispatch, mas_test.cpp
• mas_test_old.cpp keeps boost:: aliases as the deprecated-API test
• Docs: added inline MAS Visitor concept + adaptor sections, linked references, deprecation warning...

Motivation

We should progressively deprecate what is in the global boost namespace.

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.

[cppalliance-bot]

An automated preview of the documentation is available at https://572.graph.prtest3.cppalliance.org/libs/graph/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-08-27 10:15:21 UTC

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 33062001653 vs develop run 33052604369 (aea6b04f3c).

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

No header-inclusion-weight changes.

Transitive Boost modules: 66 → 66 (0)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 33062001547 vs develop run 33052604288 (aea6b04f3c).

Job	Baseline	After	Delta
macos (clang, 14)	647	647	0
macos (clang, 17)	610	610	0
macos (clang, 20)	610	610	0
ubuntu (clang-19, 14)	647	647	0
ubuntu (clang-19, 17)	610	610	0
ubuntu (clang-19, 20)	610	610	0
ubuntu (clang-19, 23)	610	610	0
ubuntu (gcc-14, 14)	795	795	0
ubuntu (gcc-14, 17)	931	931	0
ubuntu (gcc-14, 20)	931	931	0
ubuntu (gcc-14, 23)	931	931	0
windows_msvc_14_3 (msvc-14.3)	952	952	0