fix(polyline): keep wide-polyline fills precise at UTM coordinates

The solid fill band of a wide LwPolyline / Polyline2D was built from
absolute WCS vertex coordinates cast straight to f32, then handed to a
HatchModel with world_origin = [0, 0]. At UTM-scale coordinates (~1e6)
f32 resolves to ~0.06, so the half-width offset collapsed and the
relative-to-eye fill shader cancelled `boundary - eye` to noise — the
band rendered as a string of squares. (Exploding the polyline drops the
width, hence no band and a clean line, which is why that "fixed" it.)

Build the band relative to a world_origin (the first vertex) in f64 and
store the small f32 offsets, mirroring how baked hatch fills keep
precision. Same class of bug as the on-create hatch-fill fix.

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

src/scene/mod.rs

@@ -4763,9 +4763,15 @@ impl Scene {
 
         // Wide LWPolyline and Polyline2D fills
         for entity in self.document.entities() {
-            let (common, fills) = match entity {
-                EntityType::LwPolyline(pl) => (&pl.common, crate::entities::lwpolyline::wide_fills(pl)),
-                EntityType::Polyline2D(pl) => (&pl.common, crate::entities::polyline::wide_fills(pl)),
+            let (common, fill_origin, fills) = match entity {
+                EntityType::LwPolyline(pl) => {
+                    let (o, f) = crate::entities::lwpolyline::wide_fills(pl);
+                    (&pl.common, o, f)
+                }
+                EntityType::Polyline2D(pl) => {
+                    let (o, f) = crate::entities::polyline::wide_fills(pl);
+                    (&pl.common, o, f)
+                }
                 _ => continue,
             };
             if fills.is_empty() {
@@ -4792,7 +4798,7 @@ impl Scene {
                     color,
                     angle_offset: 0.0,
                     scale: 1.0,
-                    world_origin: [0.0; 2],
+                    world_origin: fill_origin,
                     vp_scissor: None,
                     draw_depth: depth_map.get(&common.handle.value()).copied().unwrap_or(0.0),
                 });