For the next session, since this one is about to be cleared. Current
design only -- `Scroll` owns the position, the gesture and the fling; a
child is either moved or answers `Widget::scrolls_itself` and is handed
deltas; one sign convention, the finger's. It carries the things that are
expensive to rediscover and easy to undo by accident: why the two `&self`
capability methods must not be `&mut` (`get_dyn_mut` marks dirty), why
`scroll_offset` exists beside `apply_scroll`'s remainder, why the
measuring draw is free, why nothing is marked by hand, why the height
cache stays in the container, why the transcript builds its `Scroll` by
hand instead of through `.scrollable_to_end()`, and the measured numbers
behind "a `LazySpan` is not a `Span`".
Also names the one thing still open -- the pin -- with the two ways to
close it and an instruction to ask Iris rather than guess.
`scroll.rs` and `lazy_span.rs` now point at it from their module docs
rather than restating it, AGENTS.md lists it beside the other design
documents, and IRIS_TODO.md's in-progress entry defers to it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
