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/mask.rs

@@ -6,8 +6,7 @@ pub struct Masked {
 
 impl Widget for Masked {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let own = painter.placement();
-        painter.set_mask(own);
+        painter.set_mask(DrawRegion::Extent(UiRegion::FULL));
         painter.widget(&self.inner);
         // What it occupies is its box, on both axes, for the reason `Scroll`
         // reports the same: it clips what is inside to that box, so it can

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);

src/widget/position/span.rs

@@ -37,7 +37,7 @@ impl Widget for Span {
             // from the cursor, because a text has to wrap at the width
             // actually there.
             let room = axis.pair(Some(along(cursor, far)), None);
-            let len = match painter.known_len(child, axis, region) {
+            let len = match painter.known_len(child, axis, region, room) {
                 Some(len) => len,
                 None => painter.widget_at(child, region, room).len(axis),
             };

src/widget/text/mod.rs

@@ -89,8 +89,7 @@ impl TextView {
         // hair under that line, and the break made in it is not the break a
         // cold layout makes there.
         let size = Size::from_px(PxVec2::ceil_from_f32(tex.size));
-        let within = region.within(&painter.placement());
-        painter.glyphs(tex, within);
+        painter.glyphs(tex, DrawRegion::Extent(region));
         (region, size)
     }