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/shaders/hatch.wgsl

@@ -274,6 +274,11 @@ fn check_family(
         let proj = v.xz.x * inst.grad_cos + v.xz.y * inst.grad_sin;
         let t = clamp((proj - inst.grad_min) / inst.grad_range, 0.0, 1.0);
         return mix(inst.color, inst.color2, t);
+    } else if inst.mode == 3u {
+        // Radial gradient: centre is (grad_cos, grad_sin), radius is grad_range.
+        let d = length(v.xz - vec2<f32>(inst.grad_cos, inst.grad_sin));
+        let t = clamp(d / inst.grad_range, 0.0, 1.0);
+        return mix(inst.color, inst.color2, t);
     }
 
     // 3. Pattern LOD: when the densest family's spacing projects below

src/shaders/hatch_web.wgsl

@@ -228,6 +228,11 @@ fn check_family(
         let proj = v.xz.x * h.grad_cos + v.xz.y * h.grad_sin;
         let t    = clamp((proj - h.grad_min) / h.grad_range, 0.0, 1.0);
         return mix(h.color, h.color2, t);
+    } else if h.mode == 3u {
+        // Radial gradient: centre is (grad_cos, grad_sin), radius is grad_range.
+        let d = length(v.xz - vec2<f32>(h.grad_cos, h.grad_sin));
+        let t = clamp(d / h.grad_range, 0.0, 1.0);
+        return mix(h.color, h.color2, t);
     }
 
     // Pattern LOD: when the densest family projects below 2 px, lines blur into