Record frame-extent performance gains and alignment verification

docs/PLAN.md

@@ -876,7 +876,10 @@ so reuse composes positions in the same order as a fresh draw instead of
 recovering fractions by division. The experimental oracle requires exact
 pixel-region equality. `Widget::draw` remains the only layout body. The
 prototype has significant container-update costs and is not yet adopted into
-the pinned framework.
+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.
 
 **A `Span` leaves its children in the offered orthogonal region unless
 explicitly compacted** (2026-09-12). It still reports the widest orthogonal