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/include/layout.hpp

@@ -138,18 +138,38 @@ struct MobiusMap {
 /// per-vertex UV coordinates plus a per-half-edge UV atlas for seamed
 /// textures.
 struct Layout2D {
-    /// uv[v.idx()] — primary 2-D position (first / shallowest-BFS-depth visit).
+    /// Primary 2-D position per vertex (first / shallowest-BFS-depth visit).
+    ///
+    /// **Indexing:** `uv[v.idx()]` — indexed by the raw integer vertex index.
+    /// **Length:** `mesh.number_of_vertices()`.
+    ///
+    /// \pre  No vertices have been removed from `mesh` after loading (i.e.
+    ///       `mesh.is_valid()` and no compaction was performed).  On a fresh
+    ///       `Surface_mesh` loaded from file, `v.idx()` is always in
+    ///       `[0, number_of_vertices())` and contiguous.  If vertices were
+    ///       deleted and `mesh.collect_garbage()` was called, re-run the
+    ///       layout — indices will have shifted.
+    ///
+    /// Access pattern:
+    /// ```cpp
+    /// for (auto v : mesh.vertices())
+    ///     Eigen::Vector2d uv_v = layout.uv[v.idx()];
+    /// ```
     std::vector<Eigen::Vector2d> uv;
 
-    /// halfedge_uv[h.idx()] — UV of source(h) as seen from face(h).
+    /// UV of `source(h)` as seen from `face(h)`, indexed by `h.idx()`.
     ///
-    /// For interior (non-seam) halfedges: equals uv[source(h).idx()].
-    /// For seam halfedges:  carries the UV from the virtual unfolding across
-    ///   the cut (i.e. the trilaterated position that was NOT used as the
-    ///   primary uv).  This gives each face its own copy of a seam vertex,
-    ///   enabling a proper GPU texture atlas without vertex duplication.
+    /// **Indexing:** `halfedge_uv[h.idx()]` — raw integer halfedge index.
+    /// **Length:** `mesh.number_of_halfedges()`.  Same no-compaction precondition
+    /// as `uv` (see above).
     ///
-    /// Size = mesh.number_of_halfedges().  Border halfedges = (0,0).
+    /// For interior (non-seam) halfedges: equals `uv[source(h).idx()]`.
+    /// For seam halfedges: carries the UV from the virtual unfolding across the
+    ///   cut — the trilaterated position that was NOT used as the primary `uv`.
+    ///   This gives each face its own copy of a seam vertex, enabling a proper
+    ///   per-halfedge GPU texture atlas without vertex duplication.
+    ///
+    /// Border halfedges (outer face) hold `(0, 0)`.
     std::vector<Eigen::Vector2d> halfedge_uv;
 
     bool success  = false; ///< `true` iff the BFS placed every vertex.