feat(gradient): selectable gradient types + working creation

GRADIENT silently degraded to a plain hatch: add_hatch rebuilt the
render model from the DXF entity but never encoded the gradient on it,
so the committed fill lost its gradient entirely. The entity now
carries the full gradient record (name, angle, colour stops,
solid-fill flag), and the standard shape set became selectable:
Linear / Cylinder / Spherical / Hemispherical / Curved plus an Invert
toggle, offered as prompt buttons on the GRADIENT command and as a
Gradient type dropdown + Invert box in the properties panel. All nine
standard DXF names round-trip (inverted linear persists by swapping
the colour stops), and the shaders shape each kind distinctly —
mirrored cylinder, eased curve, dome-profiled hemisphere — on both the
native and web pipelines without changing the GPU struct sizes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scene/entity.rs

@@ -1418,14 +1418,13 @@ impl Scene {
             gradient_color1 = stop(0);
             let color2 = stop(1).unwrap_or(color);
             let angle_deg = dxf.pattern_angle.to_degrees() as f32;
-            // Spherical / hemispherical / curved names are radial fills; linear
-            // and cylinder names run along the angle.
-            let n = dxf.gradient_color.name.to_ascii_uppercase();
-            let radial = n.contains("SPHERICAL") || n.contains("CURVED");
+            let (kind, invert) =
+                model::hatch_model::GradientKind::from_name(&dxf.gradient_color.name);
             model::hatch_model::HatchPattern::Gradient {
                 angle_deg,
                 color2,
-                radial,
+                kind,
+                invert,
             }
         } else if dxf.is_solid {
             model::hatch_model::HatchPattern::Solid
@@ -1885,6 +1884,50 @@ impl Scene {
         } else {
             1.0
         };
+        // A gradient fill must be encoded on the DXF entity itself: the render
+        // model is rebuilt from the entity below (`add_entity` →
+        // `hatch_model_from_dxf`), so a gradient kept only on the command's
+        // model silently degraded to a plain pattern hatch.
+        if let crate::scene::model::hatch_model::HatchPattern::Gradient {
+            angle_deg,
+            color2,
+            kind,
+            invert,
+        } = &model.pattern
+        {
+            let to_color = |c: [f32; 4]| acadrust::types::Color::Rgb {
+                r: (c[0] * 255.0).round().clamp(0.0, 255.0) as u8,
+                g: (c[1] * 255.0).round().clamp(0.0, 255.0) as u8,
+                b: (c[2] * 255.0).round().clamp(0.0, 255.0) as u8,
+            };
+            dxf.is_solid = true;
+            dxf.gradient_color.enabled = true;
+            dxf.gradient_color.name = kind.dxf_name(*invert).to_string();
+            // The render model reads the gradient angle from pattern_angle
+            // (radians); the gradient record keeps its own copy for the file.
+            dxf.pattern_angle = (*angle_deg as f64).to_radians();
+            dxf.gradient_color.angle = (*angle_deg as f64).to_radians();
+            dxf.gradient_color.is_single_color = false;
+            // Linear has no INV name in the standard set: persist an inverted
+            // linear by swapping the colour stops instead.
+            let (c0, c1) =
+                if *invert && matches!(kind, crate::scene::model::hatch_model::GradientKind::Linear)
+                {
+                    (*color2, model.color)
+                } else {
+                    (model.color, *color2)
+                };
+            dxf.gradient_color.colors = vec![
+                acadrust::entities::hatch::GradientColorEntry {
+                    value: 0.0,
+                    color: to_color(c0),
+                },
+                acadrust::entities::hatch::GradientColorEntry {
+                    value: 1.0,
+                    color: to_color(c1),
+                },
+            ];
+        }
 
         // `add_entity` already builds the render model from the DXF entity via
         // `hatch_model_from_dxf` and inserts it with a correct `world_origin`