feat(scene): border/bevel/arc prims; paint_self mirrors push_widget_vertices (Phase 3)

Add Prim::Border (rounded fill + solid stroke), Prim::Bevel (inset rounded fill + beveled edges), and Prim::Arc to the display list, with PaintCtx emit methods (offset-applied). Rework Element::paint_self to mirror the toolkit's own-geometry generator push_widget_vertices: a beveled plate, else a bordered plate, else a plain rounded background (from corner_radii + rounded_corners), then arcs, decoration quads, circles, and text. So a widget's full own appearance is now expressible in the DisplayList. Backend tessellation of these prims + visual fidelity are validated when render() is routed through the walk (next, runtime-gated). Headless tests cover emission + offset; scene suite green.

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

 src/scene/paint.rs | 43 ++++++++++++++++++++++++++++++++++++++++++-
 src/widget/mod.rs  | 30 ++++++++++++++++++++++--------
 2 files changed, 64 insertions(+), 9 deletions(-)

diff --git a/src/scene/paint.rs b/src/scene/paint.rs
index 0286291..4c6dc22 100644
--- a/src/scene/paint.rs
+++ b/src/scene/paint.rs
@@ -29,10 +29,21 @@ pub enum Cap {
 
 /// A single paint primitive in logical pixels (absolute coordinates once emitted). These mirror
 /// the toolkit's existing tessellators so a `DisplayList` maps directly onto them at draw time.
+/// Per-corner radii `(top_left, top_right, bottom_right, bottom_left)`, matching the toolkit's
+/// `CornerRadii` order.
+pub type Radii = (f32, f32, f32, f32);
+
 #[derive(Clone, Debug, PartialEq)]
 pub enum Prim {
     Quad { rect: Rect, color: [f32; 4] },
     RoundedRect { rect: Rect, radius: f32, corners: (bool, bool, bool, bool), color: [f32; 4] },
+    /// A rounded fill plus a solid border stroke — a widget's own "plate" (mirrors
+    /// `push_widget_vertices`' non-bevel branch: rounded bg + `push_plate_solid_border_vertices`).
+    Border { rect: Rect, radii: Radii, fill: [f32; 4], border: [f32; 4], thickness: f32 },
+    /// A beveled plate: an inset rounded fill plus lightened/darkened edges (mirrors
+    /// `push_widget_vertices`' bevel branch).
+    Bevel { rect: Rect, radii: Radii, color: [f32; 4], depth: f32 },
+    Arc { cx: f32, cy: f32, radius: f32, thickness: f32, start: f32, end: f32, color: [f32; 4] },
     Vector { x1: f32, y1: f32, x2: f32, y2: f32, thickness: f32, color: [f32; 4], cap: Cap },
     Circle { cx: f32, cy: f32, radius: f32, color: [f32; 4] },
     Text { text: String, x: f32, y: f32, font_size: f32, color: [u8; 3] },
@@ -163,6 +174,21 @@ impl PaintCtx {
         self.push(Prim::Circle { cx: cx + ox, cy: cy + oy, radius, color });
     }
 
+    pub fn border(&mut self, rect: Rect, radii: Radii, fill: [f32; 4], border: [f32; 4], thickness: f32) {
+        let rect = self.apply_offset(rect);
+        self.push(Prim::Border { rect, radii, fill, border, thickness });
+    }
+
+    pub fn bevel(&mut self, rect: Rect, radii: Radii, color: [f32; 4], depth: f32) {
+        let rect = self.apply_offset(rect);
+        self.push(Prim::Bevel { rect, radii, color, depth });
+    }
+
+    pub fn arc(&mut self, cx: f32, cy: f32, radius: f32, thickness: f32, start: f32, end: f32, color: [f32; 4]) {
+        let (ox, oy) = self.offset;
+        self.push(Prim::Arc { cx: cx + ox, cy: cy + oy, radius, thickness, start, end, color });
+    }
+
     pub fn text(&mut self, text: impl Into<String>, x: f32, y: f32, font_size: f32, color: [u8; 3]) {
         let (ox, oy) = self.offset;
         self.push(Prim::Text { text: text.into(), x: x + ox, y: y + oy, font_size, color });
@@ -274,9 +300,24 @@ mod tests {
         let mut ctx = PaintCtx::new();
         ctx.quad(r(0.0, 0.0, 1.0, 1.0), [0.0; 4]);
         ctx.rounded_rect(r(0.0, 0.0, 1.0, 1.0), 2.0, (true, false, true, false), [0.0; 4]);
+        ctx.border(r(0.0, 0.0, 10.0, 10.0), (2.0, 2.0, 2.0, 2.0), [0.1; 4], [0.9; 4], 1.5);
+        ctx.bevel(r(0.0, 0.0, 10.0, 10.0), (2.0, 2.0, 2.0, 2.0), [0.3; 4], 2.0);
+        ctx.arc(5.0, 5.0, 4.0, 1.0, 0.0, 3.14, [0.0; 4]);
         ctx.vector(0.0, 0.0, 10.0, 0.0, 1.0, [0.0; 4], Cap::Arrow);
         ctx.circle(5.0, 5.0, 3.0, [0.0; 4]);
         ctx.text("hi", 1.0, 2.0, 12.0, [255, 255, 255]);
-        assert_eq!(ctx.finish().len(), 5);
+        assert_eq!(ctx.finish().len(), 8);
+    }
+
+    #[test]
+    fn border_and_bevel_are_offset() {
+        let mut ctx = PaintCtx::new();
+        ctx.translate(10.0, 20.0, |ctx| {
+            ctx.border(r(0.0, 0.0, 5.0, 5.0), (1.0, 1.0, 1.0, 1.0), [0.0; 4], [1.0; 4], 1.0);
+            ctx.bevel(r(0.0, 0.0, 5.0, 5.0), (1.0, 1.0, 1.0, 1.0), [0.0; 4], 1.0);
+        });
+        let list = ctx.finish();
+        assert!(matches!(list.items[0].prim, Prim::Border { rect, .. } if rect.x == 10.0 && rect.y == 20.0));
+        assert!(matches!(list.items[1].prim, Prim::Bevel { rect, .. } if rect.x == 10.0 && rect.y == 20.0));
     }
 }
diff --git a/src/widget/mod.rs b/src/widget/mod.rs
index 812914f..2cf71cc 100644
--- a/src/widget/mod.rs
+++ b/src/widget/mod.rs
@@ -480,16 +480,30 @@ pub trait Element {
     /// are handled by the paint walk (`scene::painter`), not here.
     fn paint_self(&self, ui: &UiContext, ctx: &mut crate::scene::paint::PaintCtx) {
         use crate::scene::layout::Rect;
-        let (r1, r2, r3, r4) = self.rounded_corners();
-        if r1 || r2 || r3 || r4 {
-            let (x, y, w, h) = self.rect();
-            let c = self.color();
-            if c[3].abs() > 0.001 {
-                ctx.rounded_rect(Rect { x, y, width: w, height: h }, self.corner_radius(), (r1, r2, r3, r4), c);
+        let (x, y, w, h) = self.rect();
+        let rect = Rect { x, y, width: w, height: h };
+        let color = self.color();
+        let cr = self.corner_radii();
+        let radii = (cr.top_left, cr.top_right, cr.bottom_right, cr.bottom_left);
+
+        // The widget's own plate — mirrors `push_widget_vertices`: a beveled plate, or a bordered
+        // plate, or (the simple case) a rounded background from `rounded_corners`.
+        if let Some(depth) = self.plate_bevel() {
+            ctx.bevel(rect, radii, color, depth);
+        } else if let Some((border_color, thickness)) = self.solid_border() {
+            ctx.border(rect, radii, color, border_color, thickness);
+        } else if color[3].abs() > 0.001 {
+            let (r1, r2, r3, r4) = self.rounded_corners();
+            if r1 || r2 || r3 || r4 {
+                ctx.rounded_rect(rect, self.corner_radius(), (r1, r2, r3, r4), color);
             }
         }
-        for (x, y, w, h, c) in self.all_quads(ui) {
-            ctx.quad(Rect { x, y, width: w, height: h }, c);
+
+        for (qx, qy, qw, qh, c) in self.all_quads(ui) {
+            ctx.quad(Rect { x: qx, y: qy, width: qw, height: qh }, c);
+        }
+        for (cx, cy, r, t, start, end, c) in self.extra_arcs() {
+            ctx.arc(cx, cy, r, t, start, end, c);
         }
         for (cx, cy, r, c) in self.extra_circles() {
             ctx.circle(cx, cy, r, c);