Reconcile `labeled_graph` and `named_graph` into a single documented component

[Becheler]

Problem

Boost.Graph has two overlapping mechanisms for label to vertex lookup:

• labeled_graph (Sutton, 2009): simple API, external adaptor, but undocumented, non-compilable example, and broken in multiple ways :
  • No add_edge in labeled_graph #147
  • labeled_graph::remove_vertex does not remove label #167
  • Different behavior labeled_graph::add_vertex()/labeled_graph::insert_vertex() with label as string and as integer. #187
• named_graph (Gregor, 2007): sound internals (multi-index), but requires trait specialization, tightly coupled to adjacency_list, drags in Boost.MultiIndex unconditionally, which drags mp11 ... also undocumented. Friction to adoption due to boilerplate code is high.

Both are incomplete. Users end up maintaining their own std::map<Label, vertex_descriptor> because both options are either broken or too cumbersome.

Proposal

• keep labeled_graph external API (simpler one):
  • add_vertex(label, g)
  • g.vertex(label)
  • remove_vertex(label, g)
• Replace labeled_graph internal _map with named_graph multi-index machinery. This would eliminate the entire set of map-sync bugs by construction.
• Decouple named_graph from adjacency_list so it can back any graph type
• Deprecate direct use of named_graph traits (internal_vertex_name, internal_vertex_constructor) as they become implementation details
• Drop vecS label storage that seems fundamentally broken and blocked by static_assert in Fix: labeled_graph pointer specialization remove_vertex not removing label #464)
• Reverse lookup (vertex to label)
• Eliminate the owning/pointer graph duplication from labeled_graph
• Write a documentation page + working examples