Record retained extent-frame progress and remaining layout gap

docs/PLAN.md

@@ -879,7 +879,14 @@ prototype has significant container-update costs and is not yet adopted into
 the pinned framework. A container's length query can evaluate the offered extent
 without first placing the child's reported size; the parent's subsequent draw
 assigns the final slot using the same `Widget::draw` body. A parent-selected slot
-still honors a fixed child's alignment within that slot.
+still honors a fixed child's alignment within that slot. Child frames can also
+retain an extent-relative region through `widget_within(DrawRegion::Extent)`;
+padding and stacks use this instead of baking the current placement into child
+coordinates. Pixel insets carry numeric validity through the extent. General
+fractional endpoints and fractional reports retain conservative placement
+dependencies; fixed-point composition must not be reassociated to remove them.
+Retained movement preserves the slot chosen from measurement, even if the final
+draw reports another size.
 
 **A `Span` leaves its children in the offered orthogonal region unless
 explicitly compacted** (2026-09-12). It still reports the widest orthogonal