Say which box a pin reaches the parent through, and derive the offer

The review pass over the two commits before it:

- `LayoutHolds`'s comment said a pin does not compose into the parent. It
  does, where the box it pinned is the parent's own box, which is the one
  case where the parent's own length is what was pinned.
- `DrawInfo::offer` was stored beside the two fields it is computed from.
  It is a method now, with the open question written where it is asked
  rather than only in the handoff.
- `Painter::own` is `frame_own` beside `extent_own`, since a widget's own
  box is the extent and the frame is what it is a part of.
- One expression for the length a rule gives a frame (`narrowed_by`) and
  one for what a widget answered (`ActiveData::measured`), each of which
  had two.
- The counter said "the placement it was pinned to" for what is now a
  length; the deferral in `redraw` named the seeds that made it necessary
  before the last commit rather than the ones that do now; three comments
  claimed an inset that does not exist yet.

No behaviour change: 108 suite tests, 20 core, the 11 generated cases, the
same two shrinker seeds failing at 400/5, and `view` and `tabs` byte-identical
to the renders taken before it.

core/src/ui/active.rs

@@ -17,7 +17,7 @@ pub struct ActiveData {
     pub extent: UiRegion,
     /// That frame in its parent's frame coordinates, before composition:
     /// forwarded whole by a transparent container, narrowed by a declared
-    /// length or an inset. Its length is the same on every ask, which is what
+    /// length. Its length is the same on every ask, which is what
     /// a local redraw relies on to ask its parent's own question again.
     pub frame: UiRegion,
     /// What of its parent's extent the drawing was given, and what it was
@@ -77,6 +77,14 @@ pub struct ActiveData {
 }
 
 impl ActiveData {
+    /// What it answered when its parent measured it, where it has been
+    /// measured at all. Not `size`, which is what its last drawing reported:
+    /// a drawing made in the box that answer chose is answering a different
+    /// question.
+    pub fn measured(&self) -> Option<Size> {
+        self.answer.map(|(size, _)| size)
+    }
+
     /// Whether what it answered still stands for a frame of these pixel
     /// lengths. The answer was given in the box its parent first asked
     /// about, which is what it is checked against -- `holds` on the record