feature: support stateful visitors for neighbor_bfs and biconnected

[Becheler]

Important

New algorithms and API additions need a design issue first. Such a PR is not reviewed until an issue has settled the API and frozen the scope. PRs whose scope grows under review go stale. See How To Contribute an Algorithm.

Motivation

Extends #614 #634 #635 #636 #637 to two new algorithms:

• biconnected_components
• neighbor_bfs

What this PR does

• add std::ref support for stateful visitors
• add required positional overload
• add doc
• add missing test coverage

Before submitting

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

Kind of change

Tick exactly one. Each kind is reviewed differently, so mixing them in one PR makes the diff difficult to review (why).

• Reformat (layout only, no token added or removed)
• Refactor (no observable behavior change)
• Behavior change (observable to users of the library)
• No code change (documentation, build, CI, or tooling)

Change category

• Bug fix
• New algorithm or API addition
• Deprecation or removal
• Performance
• Tests
• Documentation
• Build, CI, or tooling
• Other (specify below)

[cppalliance-bot]

An automated preview of the documentation is available at https://639.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.

2026-10-10 10:04:35 UTC

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 38043350865 vs develop run 37951267968 (02ebe3046d).

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

No header-inclusion-weight changes.

Transitive Boost modules: 47 → 47 (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 38043350859 vs develop run 37951268000 (02ebe3046d).

Job	Baseline	After	Delta
macos (clang, 14)	253	253	0
macos (clang, 17)	252	252	0
macos (clang, 20)	252	252	0
ubuntu (clang-19, 14)	253	253	0
ubuntu (clang-19, 17)	252	252	0
ubuntu (clang-19, 20)	252	252	0
ubuntu (clang-19, 23)	252	252	0
ubuntu (gcc-14, 14)	349	349	0
ubuntu (gcc-14, 17)	345	345	0
ubuntu (gcc-14, 20)	345	345	0
ubuntu (gcc-14, 23)	345	345	0
windows_msvc_14_3 (msvc-14.3)	937	937	0