Fix: labeled_graph pointer specialization remove_vertex not removing label

[Becheler]

Fixes #167

• labeled_graph.hpp : pointer specialization remove_vertex now uses graph_detail::remove_labeled_vertex(_map, *_graph, l). This fixes labeled_graph::remove_vertex does not remove label #167 as label was not removed from _map
• labeled_graph.hpp both specializations (graph owning and non-owning): static_assert blocking remove_vertex for vecS label storage. When remove_vertex called vector::erase, it shifts all indices of the labels after the erase point. It was preferred over documentation-only (silent corruption) or runtime error (damage already done before check or pay for a check before every vector::erase call) because the combination is fundamentally broken for any non-trailing removal.
• labeled_graph.cpp: added test_remove_vertex_suite + _ptr_variant : 5 scenarios across all selector/graph/ownership combos
• labeled_graph.cpp: added test_multimap_suite + _ptr : 3 duplicate-label scenarios for multimapS/hash_multimapS, owning + pointer
• test_graphs.cpp: changed defaultS to mapS for unsigned labeled graph : defaultS picks vector, now blocked by static_assert because it invalidates labels
• compile_fail_remove_vertex_vecS.cpp: new file, verifies static_assert rejects vecS removal at compile time