fix(hatch): radial gradients and correct gradient stop colours

A gradient hatch was drawn linear from the entity colour to its second
stop, so a red->yellow spherical fill came out as a pale linear wash. Two
fixes:
- Colour: start from the gradient's first stop, not the entity colour.
  hatch_model_from_dxf already baked stop-0 in; stop synced_hatch_models
  from clobbering it with the entity's resolved colour for gradient fills.
- Shape: spherical / hemispherical / curved gradient names now render as a
  radial fill (new shader mode 3, centred on the boundary with radius to the
  farthest vertex) instead of linear. Native and web shaders both updated.

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

src/scene/pipeline/hatch_web_gpu.rs

@@ -110,9 +110,14 @@ impl HatchWebGpu {
         let (mode, color2, grad_cos, grad_sin) = match &model.pattern {
             HatchPattern::Solid => (1u32, [0.0f32; 4], 0.0f32, 0.0f32),
             HatchPattern::Pattern(_) => (0u32, [0.0f32; 4], 0.0f32, 0.0f32),
-            HatchPattern::Gradient { angle_deg, color2 } => {
-                let r = angle_deg.to_radians();
-                (2u32, *color2, r.cos(), r.sin())
+            HatchPattern::Gradient { angle_deg, color2, radial } => {
+                if *radial {
+                    // Radial: centre is the local origin; grad_cos/sin unused.
+                    (3u32, *color2, 0.0, 0.0)
+                } else {
+                    let r = angle_deg.to_radians();
+                    (2u32, *color2, r.cos(), r.sin())
+                }
             }
         };
 
@@ -181,6 +186,16 @@ impl HatchWebGpu {
                 // Floor matches the desktop hatch renderer (hatch_gpu.rs).
                 (proj_min, (proj_max - proj_min).max(1.0))
             }
+        } else if mode == 3 {
+            // Radial: range = the farthest boundary vertex from the centre.
+            let radius = model
+                .boundary
+                .iter()
+                .filter(|v| v[0].is_finite() && v[1].is_finite())
+                .map(|&[x, y]| (x * x + y * y).sqrt())
+                .fold(0.0_f32, f32::max)
+                .max(1.0);
+            (0.0, radius)
         } else {
             (0.0, 1.0)
         };