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.

src/widget/position/pad.rs

@@ -16,12 +16,10 @@ impl Widget for Pad {
         //
         // The padding goes around what it pads: the frame passes through, so
         // the inner's fractions mean what they would without it, and only
-        // the box it draws in is moved in by the pixels.
-        // A part of this widget's own box, in that box's own lengths: the
-        // padding is pixels, which are the same pixels wherever the box
-        // lands, so nothing here reads how long the box is -- and a box
-        // chosen from this widget's own answer therefore does not feed back
-        // into that answer.
+        // the box it draws in is moved in by the pixels. Said as a part of
+        // this widget's own box in that box's own lengths, so nothing here
+        // reads how long the box is -- and a box chosen from this widget's
+        // own answer therefore does not feed back into that answer.
         let inset = |lead: Px, trail: Px| {
             Place::Within(Part::Of(UiSpan::new(
                 Len::from_parts(Rel::ZERO, lead),