feature: adding overloads to accept reference wrappers

[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 #639 to new algorithms:

• planar_face_traversal
• make_connected
• make_biconnected_planar
• make_maximal_planar

What this PR does

Brings stateful visitors passed by std::reference_wrapper possible. Adds an overload to avoid conflicting with existing API (passing Visitor&). Add tests and doc.

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)

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 38711064973 vs develop run 38291727273 (988980f4a4).

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

Dependency	develop	PR	Δ
config	94	95	+1

Transitive Boost modules: 47 → 47 (0)

[cppalliance-bot]

An automated preview of the documentation is available at https://644.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.

2026-10-15 09:17:28 UTC

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 38711065039 vs develop run 38291727442 (988980f4a4).

Job	Baseline	After	Delta
macos (clang, 14)	253	255	+2
macos (clang, 17)	252	254	+2
macos (clang, 20)	252	254	+2
ubuntu (clang-19, 14)	253	255	+2
ubuntu (clang-19, 17)	252	254	+2
ubuntu (clang-19, 20)	252	254	+2
ubuntu (clang-19, 23)	252	254	+2
ubuntu (gcc-14, 14)	349	351	+2
ubuntu (gcc-14, 17)	345	347	+2
ubuntu (gcc-14, 20)	345	347	+2
ubuntu (gcc-14, 23)	345	347	+2
windows_msvc_14_3 (msvc-14.3)	937	973	+36

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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