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]>

 src/arrange.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/arrange.rs b/src/arrange.rs
index 3312b1d..201db80 100644
--- a/src/arrange.rs
+++ b/src/arrange.rs
@@ -200,7 +200,7 @@ const OFFSCREEN_MARGIN: f64 = 50.0;
 
 /// Opacity of an unfocused window's scene tree. Both are 1.0 — the same as
 /// focused — since 2026-09-03. They used to be 0.85 (overlay) and 0.90
-/// (normal), and on a translucent backplate that dimming did not read as
+/// (normal), and on a translucent root plate that dimming did not read as
 /// "dimmer": it thinned the plate's tint, so the blurred backdrop showed
 /// through with more contrast and the window looked LESS frosted than the
 /// focused one, which then appeared to gain blur on every focus click. The