feat(polyline): smooth per-segment taper for varying-width wide polylines

A wide polyline with per-vertex varying widths rendered at its widest edge
(uniform-width ones were already exact). Draw the true taper instead.

`WireModel.taper_widths` carries a per-point full band width. The wire
instance (native slim + web fat) gains per-endpoint `world_hw_a/b`, and both
wire shaders interpolate `mix(hw_a, hw_b, which_end)` so each segment's band
narrows/widens linearly — depth-tested, and the linetype dash still applies.

Alignment (widths must match the tessellated points) is solved by building
both together: a new `TruckObject::TaperedLines(points, widths)`, filled by
`common::tapered_band_points` (straight = endpoints, arc = 16 samples with the
width lerped along it). to_truck routes a varying-width polyline here —
LwPolyline before the PLINEGEN split, Polyline2D at its return — while a
uniform-width one keeps the constant-band Contour path unchanged. `world_width`
stays the widest edge as the PDF-stroke + zoom-floor fallback.

Blocks fall back to the constant-widest band (`LocalWire` carries no
taper_widths).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/text/complex_lt.rs

@@ -305,6 +305,7 @@ pub fn apply_along(
         .into_iter()
         .filter(|s| s.len() >= 2)
         .map(|pts| WireModel {
+            taper_widths: Vec::new(),
             world_width: 0.0,
             fill_is_3d: false,
             pick_tris: Vec::new(),
@@ -345,6 +346,7 @@ pub fn apply_along(
             xy = xy.max(y);
         }
         out.push(WireModel {
+            taper_widths: Vec::new(),
             world_width: 0.0,
             fill_is_3d: false,
             pick_tris: Vec::new(),