feat(properties): navigate polyline vertices from the panel

The Current Vertex row is now a ◀ / ▶ stepper (new PropValue::Stepper)
showing "i / N". Stepping it moves the panel's focus through the
polyline's vertices — Vertex X/Y and the per-vertex start/end widths show
and edit the focused vertex instead of always the first — and the focused
vertex's grip is drawn hot (filled) in the drawing so it's visible while
navigating. Focus wraps around and resets to the first vertex when the
selection changes. Adds Area and Length to the LwPolyline Geometry group
(Polyline2D already had them). Applies to LwPolyline and Polyline2D.

The focused vertex is threaded to the per-entity property builder/editor
through a dispatch thread-local (like the curve-tolerance override), so
the PropertyEditable trait signatures are unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entities/lwpolyline.rs

@@ -4,6 +4,7 @@ use truck_modeling::{builder, Edge, Point3, Wire};
 use crate::command::EntityTransform;
 use crate::entities::common::{
     edit_prop as edit, parse_f64, rectangle_grip, ro_prop as ro, square_grip,
+    stepper_prop as stepper,
 };
 use crate::entities::traits::TruckConvertible;
 use crate::scene::convert::acad_to_truck::{TruckEntity, TruckObject};
@@ -323,22 +324,37 @@ fn grips(pline: &LwPolyline) -> Vec<GripDef> {
 }
 
 fn properties(pline: &LwPolyline) -> Vec<PropSection> {
-    let v0 = pline.vertices.first();
-    let vx = v0.map_or(0.0, |v| v.location.x);
-    let vy = v0.map_or(0.0, |v| v.location.y);
-    let start_w = v0.map_or(0.0, |v| v.start_width);
-    let end_w = v0.map_or(0.0, |v| v.end_width);
+    let n = pline.vertices.len();
+    // The panel's Current Vertex focus, clamped to this polyline's range.
+    let vi = if n == 0 {
+        0
+    } else {
+        crate::scene::view::dispatch::prop_current_vertex().min(n - 1)
+    };
+    let v = pline.vertices.get(vi);
+    let vx = v.map_or(0.0, |v| v.location.x);
+    let vy = v.map_or(0.0, |v| v.location.y);
+    let start_w = v.map_or(0.0, |v| v.start_width);
+    let end_w = v.map_or(0.0, |v| v.end_width);
+    let mp = <LwPolyline as crate::entities::traits::MassPropsCalc>::mass_props(pline);
+    let vertex_label = if n == 0 {
+        "—".to_string()
+    } else {
+        format!("{} / {}", vi + 1, n)
+    };
     vec![
         PropSection {
             title: "Geometry".into(),
             props: vec![
-                ro("Current Vertex", "current_vertex", String::new()),
+                stepper("Current Vertex", "current_vertex", vertex_label),
                 edit("Vertex X", "vertex_x", vx),
                 edit("Vertex Y", "vertex_y", vy),
                 edit("Start segment width", "start_width", start_w),
                 edit("End segment width", "end_width", end_w),
                 edit("Global width", "global_width", pline.constant_width),
                 edit("Elevation", "elevation", pline.elevation),
+                ro("Area", "area", format!("{:.4}", mp.area)),
+                ro("Length", "length", format!("{:.4}", mp.perimeter)),
             ],
         },
         PropSection {
@@ -388,26 +404,33 @@ fn apply_geom_prop(pline: &mut LwPolyline, field: &str, value: &str) {
     let Some(v) = parse_f64(value) else {
         return;
     };
+    // Per-vertex edits target the vertex the panel is focused on.
+    let n = pline.vertices.len();
+    let vi = if n == 0 {
+        0
+    } else {
+        crate::scene::view::dispatch::prop_current_vertex().min(n - 1)
+    };
     match field {
         "elevation" => pline.elevation = v,
         "global_width" => pline.constant_width = v,
         "vertex_x" => {
-            if let Some(vtx) = pline.vertices.first_mut() {
+            if let Some(vtx) = pline.vertices.get_mut(vi) {
                 vtx.location.x = v;
             }
         }
         "vertex_y" => {
-            if let Some(vtx) = pline.vertices.first_mut() {
+            if let Some(vtx) = pline.vertices.get_mut(vi) {
                 vtx.location.y = v;
             }
         }
         "start_width" => {
-            if let Some(vtx) = pline.vertices.first_mut() {
+            if let Some(vtx) = pline.vertices.get_mut(vi) {
                 vtx.start_width = v;
             }
         }
         "end_width" => {
-            if let Some(vtx) = pline.vertices.first_mut() {
+            if let Some(vtx) = pline.vertices.get_mut(vi) {
                 vtx.end_width = v;
             }
         }