Spacing: the window-edge inset is the roll plus the padding

The inset from the window edge read root_plate_padding(), a run of flat
face that on a rolled root plate begins only where the roll ends; it is
now layout::root_plate_inset(), the ladder's one number for that edge
(bevel_width plus the padding), so the flat run at the rim matches the
gap between siblings on the plate.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

 src/main.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/main.rs b/src/main.rs
index 519b2ac..5cad08a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1797,9 +1797,10 @@ impl Application for DataEditorApp {
                 let root = arena.insert(LayoutBox::container(
                     Style::column().cross_align(CrossAlign::Stretch),
                 ));
-                // DE-wide plate rim padding (style.surface.root plate.padding);
-                // the pane gap is the SplitPane's, seeded from root_plate_gap.
-                let plate_pad = cce_ui::layout::root_plate_padding();
+                // The window-edge inset (the root plate's roll plus one
+                // padding); the pane gap is the SplitPane's, seeded from
+                // root_plate_gap.
+                let plate_pad = cce_ui::layout::root_plate_inset();
                 // The File dropdown nests into the window's top-left corner:
                 // equal gap to the left and top edges, so its corner_frame
                 // adjustment (below) rounds it concentric with the plate.