fix: draw the params sliders' thumb spheres (cce-ui spheres view)

The slider thumb is a Prim::Sphere, which no legacy flat view carries —
with the fill/readout now served by rounded_quads (cce-ui side), the
knob rides the new ParametersBg::spheres view, drawn after the reliefs
so it sits on the carve.

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/render.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/render.rs b/src/render.rs
index 2291771..f202bba 100644
--- a/src/render.rs
+++ b/src/render.rs
@@ -281,6 +281,11 @@ impl State {
                             pc.recess(rect(rx, ry, rw, rh), r4, rd);
                         }
                     }
+                    // The slider thumbs (Prim::Sphere — no flat view carries
+                    // them), after the reliefs so the knob rides the carve.
+                    for (scx, scy, sr, sc) in param_bg.spheres() {
+                        pc.sphere(scx, scy, sr, sc);
+                    }
                 });
             }