fix: closing-segment markers on closed polylines; case-insensitive page-setup names

- resolve_point, set_polyline_vertex and the solver's point_for_marker
  treat the marker one past the last vertex of a closed polyline as
  vertex 0, the way line_segment already did. Equal (#1366), the axis
  pre-alignment (#1365) and the transform pin (#1361) address a picked
  segment's end that way, so on the closing segment they either refused
  the pick or pinned nothing.
- Page-setup dictionary keys compare case-insensitively: page_setup_save
  replaces "site" when "Site" exists instead of adding a second entry
  (PSETUPIN counted it as replaced, #1344), delete and rename match the
  same way, and a rename that only changes case is allowed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/scene/parametric_solve.rs

@@ -87,8 +87,16 @@ impl EntityGeom {
             (EntityGeom::Line(l), 0) => Some(l.p1),
             (EntityGeom::Line(l), 1) => Some(l.p2),
             (EntityGeom::Ray(l), 0) => Some(l.p1),
-            (EntityGeom::Polyline { points, .. }, marker) if marker >= 0 => {
-                points.get(marker as usize).copied()
+            (EntityGeom::Polyline { points, closed, .. }, marker) if marker >= 0 => {
+                // The closing segment's end marker (one past the last vertex)
+                // is vertex 0 on a closed polyline, as in `line_segment`.
+                let index = marker as usize;
+                let index = if *closed && index == points.len() && !points.is_empty() {
+                    0
+                } else {
+                    index
+                };
+                points.get(index).copied()
             }
             (EntityGeom::Circle(c), -3) => Some(c.center),
             (EntityGeom::Arc(a), 0) => Some(a.start),