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/shaders/wire.wgsl

@@ -61,6 +61,9 @@ struct InstanceIn {
     @location(13) align_total:   f32,
     // World-space half-width for a wide-polyline band. 0 = normal wire.
     @location(14) world_half_width: f32,
+    // Per-endpoint world half-width for a tapered band (0 = use the constant).
+    @location(15) world_hw_a: f32,
+    @location(16) world_hw_b: f32,
 }
 
 // Draw-order depth bias: shifts clip-space z so 2D entities of different
@@ -135,10 +138,14 @@ struct VertexOut {
     // `world_half_width / world_per_pixel` (pixels) so the band tracks zoom. A
     // normal wire (world_half_width == 0) uses the screen-pixel half-width,
     // honouring the LWDISPLAY toggle (off → collapse to a 1-pixel line).
+    // A tapered band interpolates a per-endpoint world half-width across the
+    // segment; a constant band uses `world_half_width`. Both clamp to a
+    // half-pixel so a zoomed-out band stays a hairline instead of vanishing.
+    let taper_hw = mix(in.world_hw_a, in.world_hw_b, which_end);
     var hw: f32;
-    if in.world_half_width > 0.0 {
-        // Clamp to a half-pixel so a zoomed-out band never drops below a
-        // hairline (its centre-line) instead of vanishing.
+    if taper_hw > 0.0 {
+        hw = max(taper_hw / u.world_per_pixel, 0.5);
+    } else if in.world_half_width > 0.0 {
         hw = max(in.world_half_width / u.world_per_pixel, 0.5);
     } else {
         hw = select(0.5, in.half_width, u.lwdisplay_enable > 0.5);

src/shaders/wire_indexed.wgsl

@@ -44,6 +44,10 @@ struct InstanceIn {
     @location(4) distance_a: f32,
     @location(5) distance_b: f32,
     @location(6) wire_id:    u32,
+    // Per-endpoint world half-width for a tapered band (0 = use the per-wire
+    // constant `world_half_width`).
+    @location(7) world_hw_a: f32,
+    @location(8) world_hw_b: f32,
 }
 
 const DRAW_ORDER_BIAS: f32 = 0.001;
@@ -97,10 +101,15 @@ struct VertexOut {
     // by `world_half_width / world_per_pixel` (screen pixels) so the band grows
     // and shrinks with zoom. A normal wire (world_half_width == 0) uses the
     // screen-pixel half-width, honouring the LWDISPLAY toggle.
+    // A tapered band carries a per-endpoint world half-width on the instance:
+    // interpolate across the segment so the band narrows/widens smoothly. A
+    // constant band uses the per-wire `world_half_width`. Both clamp to a
+    // half-pixel so a zoomed-out band stays a hairline instead of vanishing.
+    let taper_hw = mix(in.world_hw_a, in.world_hw_b, which_end);
     var hw: f32;
-    if c.world_half_width > 0.0 {
-        // Clamp to a half-pixel so a zoomed-out band never drops below a
-        // hairline (its centre-line) instead of vanishing.
+    if taper_hw > 0.0 {
+        hw = max(taper_hw / u.world_per_pixel, 0.5);
+    } else if c.world_half_width > 0.0 {
         hw = max(c.world_half_width / u.world_per_pixel, 0.5);
     } else {
         hw = select(0.5, c.half_width, u.lwdisplay_enable > 0.5);