Document tested frame and extent prototype and remaining costs

docs/PLAN.md

@@ -864,6 +864,17 @@ such as corner radius and text's position within horizontal overflow use
 `Len`. Flexible padding participates in the same proportional allocation as a
 span rather than silently discarding its `rest` component.
 
+**The frame/extent experiment keeps measurement and geometry references separate**
+(2026-09-17, prototype only; verification and costs in `HANDOFF.md`). The parent
+chooses the reference for fractions; placing a widget's reported size does not
+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
+prototype has significant container-update costs and is not yet adopted into
+the pinned framework.
+
 **A `Span` leaves its children in the offered orthogonal region unless
 explicitly compacted** (2026-09-12). It still reports the widest orthogonal
 child as its intrinsic size, so a row nested in a column keeps its content