Decouple scrolling translation from resize readiness

Track configure-owned presentation dependencies so independent viewport translation can continue while clients resize. Preserve linked reflow waits, native snapshots, handoff deadlines and shared rendering/input coordinates.

Closes #10.

crates/ferese-layout/src/lib.rs

@@ -4,7 +4,7 @@ use std::fmt;
 
 mod scrolling;
 
-pub use scrolling::{Column, ColumnWidth, ScrollingLayout, ViewportFocusStrategy};
+pub use scrolling::{Column, ColumnWidth, ScrollingLayout, ViewportFocusStrategy, ViewportTarget};
 
 const MIN_SPLIT_RATIO: f64 = 0.05;
 const MAX_SPLIT_RATIO: f64 = 0.95;