Unify window presentation and frame-driven shell motion

Share window geometry across overview, resize snapshots and retained closes. Advance close handoffs and group retained rendering by stacking position.

Drive shell motion from surface frames, reuse settled widget content, and synchronize notification materials with each card's fade. Replace physical config keys with duration and bounce, and update documentation and regression coverage.

compositor/src/state/window_registry.rs

@@ -22,6 +22,7 @@ pub(crate) struct WindowRecord {
     pub metadata_revision: u64,
     pub resize: Option<ResizeTransaction>,
     pub focus: Option<AnimatedValue>,
+    pub shadow: Option<AnimatedValue>,
     pub dimming: Option<crate::dimming::DimAnimation>,
     pub opening: Option<AnimatedValue>,
     pub mapped_once: bool,