D3128: Align algorithm specs with §16.3.2.4 canonical element ordering (#125)

- Reorder all spec blocks to: Mandates → Preconditions → Hardened
  preconditions → Effects → Postconditions → Returns → Throws →
  Complexity → Remarks
- Add missing Mandates sections for all algorithms with concept
  constraints (BFS, DFS, Topological Sort, Dijkstra, Bellman-Ford,
  find_negative_cycle, connected_components, afforest, kosaraju, MIS,
  Jaccard, Kruskal, inplace_kruskal, Prim)
- Add Constant When for constexpr functions (Dijkstra, Bellman-Ford)
- Add Postconditions for Dijkstra and Bellman-Ford (distances/predecessors)
- Add Throws sections for BFS, DFS, Dijkstra, Bellman-Ford
- Add [[nodiscard]] annotations to Returns (Bellman-Ford, Topological
  Sort, triangle_count, directed_triangle_count)
- Add noexcept documentation for triangle_count, directed_triangle_count
- Fix BFS visitor events: remove incorrect on_edge_relaxed/not_relaxed
- Move compile-time concept constraints from Hardened preconditions to
  Mandates (inplace_kruskal permutable, Prim basic_edge_weight_function)
- Define \constantwhen macro in P1709-preamble.tex

Author: pratzl