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/shaders/hatch.wgsl

@@ -59,7 +59,7 @@ struct HatchInstance {
     family_count:    u32,
     draw_depth:      f32,          // signed (-1,1) draw-order bias; 0 = neutral
     poly_test:       u32,         // 1 = run in_polygon (fallback), 0 = skip
-    _pad1:           u32,
+    grad_kind:       u32,         // shape (0=linear,1=cyl,2=sph,3=hemi,4=curved), bit4=invert
     _pad2:           u32,
 }
 
@@ -272,12 +272,29 @@ fn check_family(
         return inst.color;
     } else if inst.mode == 2u {
         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);
+        var t = clamp((proj - inst.grad_min) / inst.grad_range, 0.0, 1.0);
+        // Shape profile: cylinder mirrors around the middle, curved eases in.
+        let k = inst.grad_kind & 15u;
+        if k == 1u {
+            t = 1.0 - abs(2.0 * t - 1.0);
+        } else if k == 4u {
+            t = t * t;
+        }
+        if (inst.grad_kind & 16u) != 0u {
+            t = 1.0 - t;
+        }
         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);
+        var t = clamp(d / inst.grad_range, 0.0, 1.0);
+        // Hemispherical shades faster near the centre (dome profile).
+        if (inst.grad_kind & 15u) == 3u {
+            t = sqrt(t);
+        }
+        if (inst.grad_kind & 16u) != 0u {
+            t = 1.0 - t;
+        }
         return mix(inst.color, inst.color2, t);
     }
 

src/shaders/hatch_web.wgsl

@@ -222,16 +222,34 @@ fn check_family(
 @fragment fn fs_main(v: VOut) -> @location(0) vec4<f32> {
     if !in_polygon(v.xz) { discard; }
 
-    if h.mode == 1u {
+    let base_mode = h.mode & 0xFFu;
+    let gk = (h.mode >> 8u) & 15u;
+    let ginv = ((h.mode >> 8u) & 16u) != 0u;
+    if base_mode == 1u {
         return h.color;
-    } else if h.mode == 2u {
+    } else if base_mode == 2u {
         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);
+        var t    = clamp((proj - h.grad_min) / h.grad_range, 0.0, 1.0);
+        // Shape profile: cylinder mirrors around the middle, curved eases in.
+        if gk == 1u {
+            t = 1.0 - abs(2.0 * t - 1.0);
+        } else if gk == 4u {
+            t = t * t;
+        }
+        if ginv {
+            t = 1.0 - t;
+        }
         return mix(h.color, h.color2, t);
-    } else if h.mode == 3u {
+    } else if base_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);
+        var t = clamp(d / h.grad_range, 0.0, 1.0);
+        if gk == 3u {
+            t = sqrt(t);
+        }
+        if ginv {
+            t = 1.0 - t;
+        }
         return mix(h.color, h.color2, t);
     }