remove BOOST_GRAPH_EVENT_STUB

[Becheler]

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

Remove BOOST_GRAPH_EVENT_STUB utility.

Motivation

BOOST_GRAPH_EVENT_STUB generates fluent do_on_xxx() builder methods on bfs_visitor, dfs_visitor and mas_visitor. Douglas Gregor added it during his postdoc in 2003 (SVN r18853) as a pre-lambda way to attach a bare function object to an event point. The problem it solved no longer exists, the feature was never adopted, and it now stands in the way of the namespace migration:

• Untested, undocumented, no usage example for 23 years.
• No known users. None of the 22 generated names, nor the macro itself are surfaced by web searches or main github users (CGAL etc)
• Obsolete by design: it was designed in a pre-lambda C++03 world.

This is also how the issue surfaced: while moving visitor classes from namespace boost to boost::graph in maximum_adjacency_search.hpp, the macro's unqualified detail::functor_to_visitor lookup started resolving into boost::graph::detail (declared in 20+ BGL headers since 2005) instead of boost::detail, failing with error: 'functor_to_visitor' is not a member of 'boost::graph::detail'. The macro could be fixed by fully qualifying the name, but that would mean patching and then maintaining an API that has had no documented user in 22 years. Deleting it is much cheaper/healthier.

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 33028175303 vs develop run 33005161720 (6170491b19).

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

No header-inclusion-weight changes.

Transitive Boost modules: 66 → 66 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 33028175333 vs develop run 33005161740 (6170491b19).

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.

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