docs(agents): restructure and correct bgl_migration_strategy.md

[pratzl]

• Fix factual contradictions: mutation support (dynamic_graph + undirected both have add/remove vertex/edge), transpose API (class not factory fn), scorecard mutation row (was 'undirected only / 40%')
• Add 'How to use this document' reader-orientation intro
• Restructure: move practical guide (§1-11) ahead of reference material; demote Algorithm Gap Analysis, Roadmap, and Scorecard to Appendices A/B/C
• Topological sort reclassified as traversal (with BFS/DFS) in Appendix A and Appendix C; traversal score updated to 100%
• Visitor row updated to reflect shipped composable adaptor toolkit (visitor_factory.hpp); score 75% -> 90%
• Property system score corrected to 100% (by design): tag-dispatch machinery was a C++03 workaround, not a missing feature
• Overall coverage re-evaluated: ~33% -> ~46%, with core/specialist split
• Appendix B: add Boost incubation strategy subsection (Boost as BGL2 to gain field usage for std::graph proposal; alias-layer packaging guidance)
• Fix proposal-voice in §8 (DOT writer): 'should leverage' -> 'leverages'
• MutableGraph concept note clarified: mutation exists as member functions