Rename the internal backplate vocabulary to root plate (cce-ui RFC 7a-4)

Mechanical rename, no behaviour change: identifiers that carried the
retired word (statics, registry slots, setters, trait methods, locals)
now say root_plate, and comment prose says root plate.

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

 CLAUDE.md   | 2 +-
 src/main.rs | 8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/CLAUDE.md b/CLAUDE.md
index 82e2ad6..9dd8d71 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -65,7 +65,7 @@ numbers, e.g. "6l pattern", "6m recipe", document them deliberately):
   widgets are walked with `paint_root_into` (shared borrows), and finally the control
   panel and images are drawn on top. There is no `view()`; text renders from the
   paint walk (`display_list_text()` returns true).
-- **No Backplate/Plate containers**: top-level widgets register **parentless** in
+- **No root-plate/Plate containers**: top-level widgets register **parentless** in
   `UiContext` (one-time `register_widget` block guarded by `widgets_registered`,
   using raw pointers — the widgets must stay owned fields of `GraphApp` so those
   pointers stay valid). The former control-panel Plate is "dissolved": its rect,
diff --git a/src/main.rs b/src/main.rs
index 34d327b..bba6b56 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -795,8 +795,8 @@ impl Application for GraphApp {
         Some(&self.ui_context)
     }
 
-    fn is_movable_backplate_at(&self, px: f32, py: f32) -> bool {
-        // Root Backplate dissolved (Phase 6m): the surface itself is the movable plate; drag
+    fn is_movable_root_plate_at(&self, px: f32, py: f32) -> bool {
+        // root plate container dissolved (Phase 6m): the surface itself is the movable plate; drag
         // anywhere a drag-blocking widget isn't.
         self.ui_context.drag_allowed_at(px, py)
     }
@@ -1108,7 +1108,7 @@ impl Application for GraphApp {
 
         let is_first_layout = !self.widgets_registered;
         if !self.widgets_registered {
-            // The root Backplate is DISSOLVED (Phase 6m recipe): top-level widgets register
+            // The root plate container is DISSOLVED (Phase 6m recipe): top-level widgets register
             // directly (parentless), the window plate is emitted below as prims, and the two
             // Plates keep their own children.
             unsafe {
@@ -1235,7 +1235,7 @@ impl Application for GraphApp {
             self.ui_context.rebuild_spatial_grid();
         }
 
-        // 1. The dissolved root Backplate's plate, then the top-level widgets walked in the
+        // 1. The dissolved root plate container's plate, then the top-level widgets walked in the
         // old child order (menu bar, dropdown row, graph canvas, control panel on top).
         let mut pc = cce_ui::scene::paint::PaintCtx::new();
         {