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/mod.rs

@@ -401,6 +401,8 @@ pub enum Message {
     ToggleGrid,
     /// Toggle orthogonal drawing constraint — F8 / ORTHO status-bar button.
     ToggleOrtho,
+    /// Toggle LWDISPLAY header flag — LWT status-bar button.
+    ToggleLineweightDisplay,
     /// Toggle polar-angle constraint — F10 / POLAR status-bar button.
     TogglePolar,
     /// Set polar tracking angle increment (right-click POLAR button).