[mtao]

Stacked on feature/controls-model. Depends on mtao/zipper#41, which is now merged. subprojects/zipper.wrap is back on main.

Changes

• MeshAttribute built from an mdspan. It takes a rank-1 mdspan (one scalar per element) or a rank-2 mdspan with extents (components, elements), with any arithmetic type, layout (including layout_stride) or accessor. Values are stored in a column-major zipper DataArray (plain data with no arithmetic semantics), which is the only source of the shape: component_count() and count() read it, so there are no separately kept counters.
• Values keep their type. Storage is one of float, double, int32 or uint32, held in a std::variant. Narrower integers widen to the 32-bit type of the same sign (e.g. uint8 becomes uint32). Access is through type(), holds<T>(), view<T>() / view<T, D>() (std::optional, empty on a type or component-count mismatch) and visit(f). The conversion goes through a zipper cast<T>().
• Typed views report why they fail. MeshAttribute::view<T, D>() and RoleBinding::view<T, D>() return std::expected<view, ViewError>. The error says whether the binding was Unbound, the values have the WrongType (with the actual stored type), or the view had the WrongComponentCount (with the actual count).
• Bindings keep the attribute's type. A RoleBinding views either the whole attribute, one selected component (a strided view into the attribute, with no copy), or the per-element magnitude. The magnitude is computed into a derived attribute: float stays float, and other types become double. type() and visit(f) expose the bound values in their own type.
• Float conversion happens at upload. VulkanMeshDrawable uploads contiguous float bindings in place and converts anything else into a staging buffer. normalize_mesh works in float from any stored type.
• MeshData::add_attribute(name, domain, mdspan) rejects shapes with 0 or more than 255 components. The flat contiguous-range overload is now a thin adapter, and it rejects data whose length isn't a multiple of the component count. Before, such data was added with only a warning.
• Stride and ordering fixes. View extents are (components, count). Scalar extraction now uses zipper: row(component) for one component and colwise().norm() for the magnitude. The previous code used a squared rowwise() norm, which reduced over the wrong axis.
• mesh_conversion passes the OBJ vertex and triangle matrices directly via as_mdspan(), which removes flatten_columns.
• Property::Vec3Value::min uses numeric_limits<float>::lowest(). min() is the smallest positive float, so negative values were blocked.

Tests

• New: mesh_attribute_from_mdspan covers column- and row-major input, type preservation (double, int32, and uint8 widened to uint32), view<T, D>, rejection of bad shapes, and strided (interleaved-buffer) input.
• New: role_binding_keeps_attribute_type covers a double position binding viewing the attribute in place, a strided uint32 component view, a double magnitude, and visit(). role_binding_view checks each ViewError reason.
• New: scalar_binding_magnitude.
• meson test: 13/13 suites pass. test_scene_graph: 476 assertions in 58 test cases.