docs: Layout2D index semantics, CLI table, CLI roadmap (U9+U10+U11)

U9 (layout.hpp + example_layout.cpp)
  Layout2D.uv and .halfedge_uv now have explicit Doxygen docs stating:
    - indexing: v.idx() / h.idx() (raw integer index)
    - length: mesh.number_of_vertices() / number_of_halfedges()
    - precondition: no vertex removal / collect_garbage() after loading
    - access pattern example in the doc comment
  example_layout.cpp: access site comment + static_cast<size_t>(v.idx())

U10 (getting-started.md)
  New 'CLI parameter reference' table (7 rows) added directly below the
  CLI usage examples; cross-references --help as the canonical source

U11 (doc/roadmap/phases.md)
  New Phase 9h 'CLI usability extensions' section inserted before Phase 10:
    9h.1  --tol / --max-iter solver-tuning params  (~30 min, no deps)
    9h.2  -g cp_euclidean / -g inversive_distance  (~2-4 h, needs 9a ✅)
  Each sub-task has effort estimate, implementation sketch, and
  acceptance criteria so a future session can pick it up cold.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

code/examples/example_layout.cpp

@@ -128,7 +128,9 @@ int main(int argc, char* argv[])
     if (layout.success) {
         std::cout << "UV coordinates:\n";
         for (auto v : mesh.vertices()) {
-            auto& p = layout.uv[v.idx()];
+            // layout.uv is indexed by v.idx() (raw integer vertex index).
+            // Valid as long as no vertices were removed/compacted after loading.
+            auto& p = layout.uv[static_cast<std::size_t>(v.idx())];
             std::cout << "  v" << v.idx()
                       << ": (" << std::fixed << std::setprecision(6)
                       << p.x() << ", " << p.y() << ")\n";