Record that a leftover share narrows the frame like px and rel

Bryan's decision of 2026-09-18, which answers the Pad question and
makes the code's leftover filter in declared_lens a bug.

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

docs/HANDOFF.md

@@ -50,6 +50,13 @@ where it stands. The step list the plan carried is gone: steps 1 to 5 are in
   too), an inset's margins, the root. A box that is an *answer* (a row's
   height, a stack sized by a child) is never anything's frame.
 - `rel` overflows on purpose when it sums past one or has pixels beside it.
+- **A `leftover` share narrows the frame exactly as a declared `px` or `rel`
+  does** (Bryan, 2026-09-18: "Leftover was always intended to narrow the
+  frame just like the other two"). The code's `declared_lens` filtering
+  `leftover` out of what narrows a child's frame is a bug, and so is the
+  wording above and in the plan that a frame is narrowed only by a declared
+  length, an inset or the root. `rel(1.0)` inside a share is the share;
+  inside a padded share it is the share less the padding, so it fits.
 - `Inset` and `Outset` take `px`, `rel` and `leftover` margins. Outset adds
   its margins to the child's report and moves the child in; Inset draws the
   child in a frame with the margins subtracted and reports the child's size
@@ -459,7 +466,18 @@ beside a `leftover` child in a horizontally scrolling row) needs its own
 decision; Compose gives the text an unbounded width there. None of this was
 run; it is what the traces and the two experiments point at.
 
-**Question 2, `Pad`.** The contradiction is not specific to `Pad`. A
+**Question 2, `Pad`.** Answered by the decision above that a share narrows
+the frame: the overflow in the example comes from the pad sitting in a
+`leftover` share whose length did not narrow the frame, and once it does,
+`rel(1.0)` inside the pad is the share less the padding and fits. `Pad`
+stays an outset. What follows for `Span` is that a `leftover` child's frame
+is only known once the room is divided, so the child cannot be settled at
+a measuring ask made in the room; it must be given its share as its frame
+when placed, which is one more reason for the single decided-box evaluation
+recommended above. The paragraph below is the earlier reading, kept for the
+record.
+
+The contradiction is not specific to `Pad`. A
 `rel(1.0)` child directly in a half-row `Fill` slot overflows its slot
 because `rel` is of the row, and Bryan accepted that. A `rel(1.0)` inside a
 pad in that slot overflows the pad's box for the same reason and by the same