fix(polyline): correct wide-polyline fill band arc center + add LWDISPLAY toggle

Wide LWPolyline/Polyline2D fill bands now sit on the centerline:
- polyline_segment_fill arc center matched explode.rs / acadrust convention
  (signed d_perp = r·(1-b²)/(1+b²), handles major arcs correctly)
- per-vertex hw0/hw1 taper applied along arc; tess density 24/circle
- synced_hatch_models subtracts hatch_offset from band boundary so it lives
  in the same offset-rel space as the wire tessellation

LWDISPLAY:
- new command `LWDISPLAY [ON|OFF]` toggles header.lineweight_display
- LWT status-bar pill (between ORTHO and POLAR) drives the same flag
- bump_geometry() on change so the viewport re-tess immediately

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/update.rs

@@ -2509,6 +2509,16 @@ impl H7CAD {
                 }
                 Task::none()
             }
+            Message::ToggleLineweightDisplay => {
+                let i = self.active_tab;
+                if i < self.tabs.len() {
+                    let h = &mut self.tabs[i].scene.document.header;
+                    h.lineweight_display = !h.lineweight_display;
+                    self.tabs[i].scene.bump_geometry();
+                    self.tabs[i].dirty = true;
+                }
+                Task::none()
+            }
             Message::TogglePolar => {
                 self.polar_mode ^= true;
                 if self.polar_mode {