Trace the scroll circularity to apply_leftover rather than the protocol

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

docs/HANDOFF.md

@@ -457,14 +457,21 @@ today fails in three widgets and is fixable locally in each:
 - `Branch` reads `extent_len(Y)` for "the rest of my box"; that is
   `Of(40px..FULL)`.
 
-`Scroll` is the one legitimate own-answer box left. With the gate gone its
-content box lays the row out afresh, the text takes the room the leftover
-child gave back, and `content_len` from the viewport measurement is stale by
-that difference; it is deterministic on both paths, so warm equals cold, but
-the scroll range is off in the circular case. That case (a wrapping text
-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.
+`Scroll` looked like the one legitimate own-answer box left, and on a
+closer read (planning pass, later the same day) its circularity is
+`Scroll`'s own doing rather than the protocol's. It resolves the content
+length with `apply_leftover`, which turns any `leftover` in the answer into a
+*whole extra viewport on top of the pixels*: a row reporting `600 px +
+leftover` in a 900 px viewport gets a 1500 px content box, so the wrapping
+text inside is offered 900 px more room than it was measured in and
+re-wraps, and the layout drifts one iteration per evaluation. Under Bryan's
+definition a share is the room left in the deciding box, so the content
+length should be `max(viewport, px + rel·viewport)`: 900 for that row, the
+rect takes the 300 px left, nothing scrolls, and the row's box in the
+content box offers every child the room it was measured in. With that the
+content box never re-offers room, `Scroll` needs no own-answer evaluation,
+and the "return the answer from the first box" rule (seed 86) has nothing
+left to guard. Proposed, not run.
 
 **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