Plot: thin lineweights keep their true width (#1749)

Every weight under 0.26 mm was clamped to the screen's 1 px floor before
the PDF and SVG exporters saw it, so 0.00, 0.09, 0.13 and 0.25 mm all
plotted alike. The floor now applies on the GPU only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/pipeline/circle_gpu.rs

@@ -264,7 +264,7 @@ pub fn extract_circle_instance_from_geom_indexed(
     }
 
     let (ch, cl) = split_ds_xyz(center[0], center[1], center[2]);
-    let hw = wire.line_weight_px * 0.5;
+    let hw = wire.line_weight_px.max(1.0) * 0.5;
     let pat0 = [
         wire.pattern[0],
         wire.pattern[1],