Say what leftover means across a span in the transparent-frames plan

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -165,12 +165,20 @@ fn draw(&mut self, painter: &mut Painter) -> Size {
 }
 ```
 
-Across itself a span reports `leftover` if any child does; otherwise the
-longest child in *frame pixels* among `px` and `rel` children, reporting
-that child's own `Len`. Comparing in pixels is safe here because the frame
-is decided from above and nothing feeds back; the read is a `frame_px_len`
-and narrows the frame range, and at the crossover both candidates are the
-same number of pixels, so the drawing is the same on either side of it.
+Across itself a span reports the longest child in *frame pixels* among the
+`px` and `rel` parts of its children's reports, as that child's own `Len`.
+Comparing in pixels is safe here because the frame is decided from above
+and nothing feeds back; the read is a `frame_px_len` and narrows the frame
+range, and at the crossover both candidates are the same number of pixels,
+so the drawing is the same on either side of it. A child's `leftover`
+across a span contributes nothing to that: across, children overlap rather
+than divide anything, so a share can only mean "as tall as the span", and
+`Place::Within(None)` already fills the extent for a leftover answer. Only
+when no child has a `px` or `rel` part does the span report `leftover`
+itself and take its parent's room (Bryan, 2026-09-18: "returning rest if
+any have it is probably fine", refined to this because it costs nothing).
+So `row![text, rect]` is as tall as the text with the rect filling it, and
+`row![rect, rect]` fills its column's share.
 
 The shapes the rest of the containers take (write them, they are short):