refactor(render): widgets paint themselves, not through the legacy tuple views

The viewport, the generic pane arm and paint_element drew each widget's
extra_quads and extra_circles; they call paint_self now, and the network
panel's connector dots are read from painted_prims. The splitters keep
their lit widget plate and skip their own paint, a flat capsule in the
same colour that would cover its relief.

Pixel A/B in a scale-2 shadow, the designer's saved state reset before
each run: identical with the viewport alone, with the spreadsheet and
playbar, with the params pane on its own, with the network pane, and with
the circular pane (whose own region varies run to run in the old build
as well).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

 src/render.rs | 36 ++++++++++++++----------------------
 1 file changed, 14 insertions(+), 22 deletions(-)

diff --git a/src/render.rs b/src/render.rs
index 2f22499..bac59c0 100644
--- a/src/render.rs
+++ b/src/render.rs
@@ -10,7 +10,7 @@ use crate::slots::{
     BREADCRUMB_IDX, HEADER_IDX, RIGHT_MENUBAR_IDX,
     SPREADSHEET_MENUBAR_IDX, SPREADSHEET_IDX,
     LEFT_MENUBAR_IDX, PARAM_MENUBAR_IDX, NETWORK_PANEL_IDX, PLAYBAR_IDX,
-    DIALOG_IDX,
+    DIALOG_IDX, SPLITTER1_IDX, SPLITTER2_IDX,
 };
 use crate::geometry::network_sphere_vertices_with_errors;
 use cce_ui::scene::layout::Rect;
@@ -491,12 +491,7 @@ impl State {
         } else if idx == VIEWPORT_IDX {
             // The scene viewer's lip is the window's own edge.
             self.append_window_lip(pc);
-            for (qx, qy, qw, qh, qc) in w.extra_quads() {
-                pc.quad(rect(qx, qy, qw, qh), qc);
-            }
-            for (cx, cy, cr, cc) in w.extra_circles() {
-                pc.circle(cx, cy, cr, cc);
-            }
+            w.paint_self(&self.ui_context, pc);
         } else if idx == CONTENT_IDX {
             // No plate here, and none behind: the network has no plate
             // (since 2026-10-06), its nodes standing on the scene. Until 2026-09-20 this arm ALSO painted
@@ -629,9 +624,13 @@ impl State {
                 }
             });
 
-            for (cx, cy, cr, cc) in w.extra_circles() {
-                if cx >= clip.x && cx <= clip.x + clip.width && cy >= clip.y && cy <= clip.y + clip.height {
-                    pc.circle(cx, cy, cr, cc);
+            // The connector dots: the circles the graph paints (the rest of it is drawn
+            // above, in this pane's own order).
+            for prim in w.painted_prims() {
+                if let cce_ui::scene::paint::Prim::Circle { cx, cy, radius: cr, color: cc } = prim {
+                    if cx >= clip.x && cx <= clip.x + clip.width && cy >= clip.y && cy <= clip.y + clip.height {
+                        pc.circle(cx, cy, cr, cc);
+                    }
                 }
             }
 
@@ -806,12 +805,10 @@ impl State {
             if idx != NETWORK_PANEL_IDX {
                 append_widget_plate_radii(w, pc, self.plate_focus_tint(idx), self.pane_plate_radii(wx, wy, ww2, wh2));
             }
-
-            for (qx, qy, qw, qh, qc) in w.extra_quads() {
-                pc.quad(rect(qx, qy, qw, qh), qc);
-            }
-            for (cx, cy, cr, cc) in w.extra_circles() {
-                pc.circle(cx, cy, cr, cc);
+            // A splitter IS its widget plate, lit in the splitter's colour; its own paint is
+            // that colour again as a flat capsule, which would cover the plate's relief.
+            if idx != SPLITTER1_IDX && idx != SPLITTER2_IDX {
+                w.paint_self(&self.ui_context, pc);
             }
         }
 
@@ -853,12 +850,7 @@ impl State {
         }
 
         append_widget_plate(element, pc);
-        for (qx, qy, qw, qh, qc) in element.extra_quads() {
-            pc.quad(rect(qx, qy, qw, qh), qc);
-        }
-        for (cx, cy, cr, cc) in element.extra_circles() {
-            pc.circle(cx, cy, cr, cc);
-        }
+        element.paint_self(&self.ui_context, pc);
 
         for child_ptr in self.ui_context.tree.children_ptrs(element.base().id()) {
             unsafe {