Fix lofted-airfoil rib placement for +spanwise section order (#256)

`airfoil_skin_geometry` chose which panel edge to loft each section's
contour onto with `plus_edge = i <= n_panels ? !increasing : increasing`,
where `increasing` tests whether the refined sections run along +spanwise.
But panels are always built with `refined_sections[p]` as corner 1/2 and
`refined_sections[p+1]` as corner 4/3 (`init_pos!`), independent of span
order, so the correct edge is purely index-based: section `i` sits on
panel `i`'s first edge for `i <= n_panels`, and on the last panel's second
edge for the final section.

For a wing whose sections run −y→+y (`increasing == true`) the old
expression inverted every mapping, drawing each section's contour at its
neighbour's station: the first-section tip got no rib and the last-section
tip got a doubled (folded) rib, while the VSM panels rendered correctly.
Replace it with `first_edge = i <= n_panels`, which reproduces the previous
(correct) result for −spanwise wings and fixes the +spanwise case.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 1-Bart-1

ext/VortexStepMethodMakieExt.jl

@@ -136,9 +136,6 @@ function airfoil_skin_geometry(body; R_b_w=nothing, T_b_w=nothing)
         n = length(sections)
         n_panels = n - 1
         n_panels < 1 && continue
-        spanwise = Point3f(wing.spanwise_direction)
-        increasing = dot(Point3f(sections[n].LE_point) -
-                         Point3f(sections[1].LE_point), spanwise) > 0
         for (i, section) in enumerate(sections)
             isnothing(section.section_aero) && continue
             panel_idx = panel_offset + min(i, n_panels)
@@ -147,9 +144,9 @@ function airfoil_skin_geometry(body; R_b_w=nothing, T_b_w=nothing)
             corners = panel.corner_points
             corner1 = Point3f(corners[:, 1]); corner3 = Point3f(corners[:, 3])
             corner2 = Point3f(corners[:, 2]); corner4 = Point3f(corners[:, 4])
-            plus_edge = i <= n_panels ? !increasing : increasing
-            leading = plus_edge ? corner1 : corner4
-            trailing = plus_edge ? corner2 : corner3
+            first_edge = i <= n_panels
+            leading = first_edge ? corner1 : corner4
+            trailing = first_edge ? corner2 : corner3
             chord = trailing - leading
             chord_len = norm(chord)
             chord_len < 1e-9 && continue