[mtao]

MeshData combined render-agnostic mesh data with one specific render setup: the position/normal/scalar roles and render state of the built-in lit, colour-mapped shader. This PR separates the two: geometry becomes a shared resource, and how an object renders it becomes a feature. Other shading contexts (textured, points, volumes, …) can later use the same geometry, and several objects can show one geometry.

New types

• MeshGeometry (a plain class, not a feature) holds named, typed attributes on vertices/edges/faces. Its version() bumps on any geometry change. It knows nothing about rendering. owns(handle) is public so shadings can check that an attribute belongs to it.
  • Topology is stored as attributes: .face_vertices (Face domain, uint32, 3 × F) and .edge_vertices (Edge domain, 2 × E, derived from the triangles unless set explicitly). The face and edge counts are those attributes' counts, so face/edge attributes have a single source for their element count; a mismatch is kept but logged as a warning.
  • Reserved names: names starting with . are reserved. The public add_attribute/remove_attribute reject them, so topology changes only through set_triangle_indices / set_edge_indices.
  • Typed index views: triangle_indices() / edge_indices() return column-major mdspan<const uint32_t, extents<K, dyn>> views (one column per element), so the shape is in the type.
  • Copying: it isn't copyable, since a default copy would share attribute instances; copy_from makes a deep copy.
  • Converting index input: input in either orientation and any integer type is converted with zipper (cast, plus transpose for N × K) instead of a hand-written copy loop. A static extent of K decides the orientation; rows win only when both extents are dynamic.
• MeshInstance (feature) holds a std::shared_ptr<MeshGeometry>: the geometry an Object shows. Several objects can show the same geometry, and the geometry outlives them. set_geometry switches it.
• BasicShading (feature) holds the position/normal/scalar role assignments and MeshRenderState. It reads its geometry through the Object's MeshInstance (geometry()).
  • Lazy resolution: bindings are resolved against the geometry whenever the geometry's version changes or the instance shows different geometry. Attributes replaced in place are followed, removed attributes unbind their roles, and magnitude bindings are recomputed. This replaces the inline bookkeeping MeshData did when attributes were added or removed.
  • Auto-binding: a role you haven't set yourself binds vertex_positions / vertex_normals automatically. A role you explicitly cleared stays cleared when the geometry changes.
  • Versioning: version() bumps only on role changes. Geometry changes are covered by MeshGeometry::version().
  • Missing geometry: without a geometry (no MeshInstance, an instance with no geometry, or not attached to an Object yet), all roles are unbound and assignments fail.
• MeshAttribute/MeshAttributeHandle and RoleBinding move to their own headers. RoleBinding gains whole / component / magnitude factories.

Callers

• VulkanMeshDrawable re-uploads buffers when either version changes, and reads render state from BasicShading.
• MeshScene::add_mesh creates an Object with a MeshInstance (showing a new, empty geometry), a BasicShading and a drawable. ensure_drawable applies to objects with a MeshInstance, and adds a BasicShading if one is missing.
• Control panels: PanelContext::active_mesh_data() is replaced by active_geometry() / active_shading().
  • The Data and Material panels edit the shading; their attribute lists come from the shading's geometry.
  • The Object tab's Mesh info shows the vertex count from the shading and the triangle/edge counts from the geometry.
• to_mesh_data becomes to_mesh_geometry. normalize_to_unit_box takes the BasicShading.
• The Qt and ImGui scene widgets and both viewer tools are updated.
• Duplicate in the ImGui scene panel now works. Before, the copy had empty geometry and never got a drawable, so it didn't render.
  • The copy shows the same geometry (instancing), with its own shading copied by BasicShading::copy_from. That copies the render state and role assignments; each assigned role points at the same-named attribute of the copy's geometry, and explicitly cleared roles stay cleared.
  • The copy and its descendants get drawables through MeshScene::ensure_drawable.
  • MeshGeometry::copy_from (a deep copy with new attribute instances) is available when an independent copy is wanted.
• Small addition: AbstractFeature::is_attached().
• magnitude_of evaluates colwise().norm() directly into an owning vector now that colwise()/rowwise() don't work on temporary expressions zipper#42 is fixed.
• The attribute role pickers in the control panels skip reserved (topology) attributes.

Tests

• Pure geometry tests (edges, topology, index layouts, attribute storage) use a bare MeshGeometry. Role tests use an Object with both features.
• New coverage:
  • geometry and shading version counters bump independently
  • a cleared role stays cleared after geometry changes
  • a shading without geometry, and geometry added to its Object later
  • attributes from another geometry are rejected
  • a magnitude binding is recomputed when its attribute is replaced, and unbinds on removal
  • normalization works from double positions
  • copy_from produces independent attributes, re-targets roles by name, and keeps cleared roles cleared
  • two objects sharing one geometry with independent role bindings; geometry changes visible through both; the geometry outliving its objects; switching an instance to other geometry
  • topology stored as attributes: domains, types and shapes, rejection of reserved names, and copying
  • a static 3 × 3 index input treats rows as triangles; a 2 × N input with N = 2 keeps one column per edge
• meson test: 13/13 suites pass. test_scene_graph: 547 assertions in 62 test cases.