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/ui/statusbar.rs

@@ -48,6 +48,8 @@ impl StatusBar {
         scale_popup_open: bool,
         // True when the scale pill is interactive (always model space; paper space only when a viewport is active/selected).
         scale_pill_enabled: bool,
+        // LWDISPLAY header flag — controls lineweight visibility in the viewport.
+        lineweight_display: bool,
     ) -> Element<'a, Message> {
         let menu_btn = button(text("≡").size(14).color(ICON_COLOR))
             .on_press(Message::Command("MENU".into()))
@@ -103,6 +105,10 @@ impl StatusBar {
                 toggle_pill("ORTHO", ortho_mode, Message::ToggleOrtho),
                 "Orthogonal Mode\nF8"
             ),
+            tip(
+                toggle_pill("LWT", lineweight_display, Message::ToggleLineweightDisplay),
+                "Show Lineweight\nLWDISPLAY"
+            ),
             polar_pill(polar_mode, polar_increment_deg),
             tip(
                 toggle_pill("DYN", dyn_input, Message::ToggleDynInput),