Record exact layout equality and measured reuse improvements

docs/PLAN.md

@@ -871,7 +871,10 @@ replace that reference. A measurement also records the available extent, so
 reusing it checks both inputs. Retained primitive and mask coordinates name
 whether they follow the frame or the extent. Numeric extent reads constrain
 layout validity; merely drawing in an extent does not require another widget
-evaluation when it moves. `Widget::draw` remains the only layout body. The
+evaluation when it moves. Widgets also retain their original local frames,
+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.