fix(plot): scale wide polyline band width into paper units through viewports

`viewport_content_wires` projects model-space wires into paper coordinates
for plotting: the points and the linetype stations are multiplied by the
viewport scale, but `world_width` (the band width of a wide polyline) was
copied unchanged. The PDF / print exporter strokes that band at
`world_width` in paper millimetres, so a 15-unit-wide bus bar seen through
a 1:100 viewport came out as a ~21 mm black blob instead of 0.15 mm, and
every single-line diagram sheet plotted as solid black shapes.

Scale `world_width` by the viewport scale alongside the points, and keep
the per-point taper widths in step (or drop them when clipping changed the
vertex count, so they cannot misalign). Negative (fixed pixel) and zero
(lineweight) widths are left untouched.

Adds a regression test that plots a 15-unit-wide polyline through a 1:10
viewport and expects a 1.5-unit band in the projected plot wires.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/scene/project.rs

@@ -321,6 +321,22 @@ impl Scene {
                 }
                 let mut out = wire.clone();
                 out.points = clipped;
+                // A wide polyline's band width is a geometric width in model
+                // units; the points were just projected into paper units, so
+                // the band (and any per-point taper) must follow the same
+                // scale or the PDF / print exporter strokes a 15-unit bus bar
+                // as 15 mm of paper. Negative values are fixed pixel widths
+                // and zero means "use the lineweight" — both stay as they are.
+                if out.world_width > 0.0 {
+                    out.world_width = wire.world_width * scale;
+                    if wire.taper_widths.len() == out.points.len() {
+                        out.taper_widths = wire.taper_widths.iter().map(|w| w * scale).collect();
+                    } else {
+                        // Clipping changed the vertex count, so the per-point
+                        // widths no longer line up; fall back to a constant band.
+                        out.taper_widths.clear();
+                    }
+                }
                 if let Some(source_length) = source_length {
                     for station in &mut clipped_stations {
                         *station *= scale;