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/scene/model/object.rs

@@ -29,6 +29,9 @@ pub enum PropValue {
     LinetypeChoice(String),
     /// Boolean flag — rendered as a toggle button (e.g. Invisible).
     BoolToggle { field: &'static str, value: bool },
+    /// A 0-based index navigated with ◀ / ▶ buttons (e.g. a polyline's Current
+    /// Vertex). `display` is the label shown between the arrows (e.g. "2 / 7").
+    Stepper { field: &'static str, display: String },
     /// Hatch pattern name — rendered as a combo_box from the catalog.
     HatchPatternChoice(String),
     /// Block attribute value keyed by its (dynamic, runtime) tag — rendered as