Retain frame and extent dependencies independently

Keep the original measurement placement separate from the assigned slot.
Validate frame and extent lengths before reusing an answer or drawing, and
represent hint-only records as having no measured answer.

Retain primitive and mask coordinates with their frame/extent reference.
Forwarded children follow a reused wrapper's placement without rerunning
valid draw bodies. Keep the single Widget::draw API.

Restore the eight failing suite cases from the region/placement prototype,
with regressions for mixed coordinate references, a changed inherited
extent, the sizing-stack fraction, and an undrawn share becoming visible.

This remains experimental: nested container updates do substantially more
work than e44dea3 despite restoring the leaf and wrapper reuse guarantees.
Do not merge it as a performance improvement.

src/widget/position/scroll.rs

@@ -15,10 +15,12 @@ impl Widget for Scroll {
         // Draw in the whole container only when its scrolling-axis length is
         // not already known, then draw it at the scrolled offset.
         let whole = UiRegion::FULL;
-        let answer_len = match painter.known_len(&self.inner, self.axis, whole) {
-            Some(len) => len,
-            None => painter.widget(&self.inner).size().axis(self.axis),
-        };
+        let own = painter.placement();
+        let answer_len =
+            match painter.known_len(&self.inner, self.axis, whole, [Some(own.x), Some(own.y)]) {
+                Some(len) => len,
+                None => painter.widget(&self.inner).size().axis(self.axis),
+            };
         let content = answer_len.apply_leftover();
         self.container_len = container_len;
         self.content_len = content.to_px(container_len);