Document the causes and correction of layout invalidation cascades

docs/PLAN.md

@@ -879,14 +879,19 @@ 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. 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.
+still honors a fixed child's alignment within that slot. Answer dependencies
+are collected only from child sizes actually read; drawing dependencies
+include all painted children. Root resize and local propagation check both
+contracts. An unchanged answer with a wider validity range retains its old
+contract: the parent's guarantee is still valid, and narrowing back must not
+trigger layout. The same rule applies to retained drawing validity. A changed
+declared size defers to its direct parent, then ordinary bottom-up propagation
+stops when both contracts remain unchanged. Blanket ancestor invalidation is
+unnecessary. The extent-relative child-frame API trial was removed: the major
+size/resize cost came from conflating these dependencies, not from Pad and
+Stack's conservative raw-placement reads. Cold layout and widespread content
+changes still perform excess provisional work; that residual has not been
+established as necessary for correctness.
 
 **A `Span` leaves its children in the offered orthogonal region unless
 explicitly compacted** (2026-09-12). It still reports the widest orthogonal