feature: add support for MAS stateful visitor

[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

• Add support for visitors passed through std::reference_wrapper.
• Examples, tests and documentation updated
• Passsing stateful visitors through std::ref is the modern norm.

Motivation

Several complaints about stateful visitors having to capture ref members.
For example it triggers static analysis warnings: https://www.reddit.com/r/cpp_questions/comments/1oj6mi6/boost_graph_library_example_seems_to_run_afoul_of/

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 33071365597 vs develop run 33069190193 (2da3c1506f).

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

No header-inclusion-weight changes.

Transitive Boost modules: 66 → 66 (0)

[cppalliance-bot]

An automated preview of the documentation is available at https://574.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 12:25:14 UTC

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 33071365589 vs develop run 33069190292 (2da3c1506f).

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.