docs: SCROLL.md, the standing reference for how iris scrolls

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>

docs/IRIS_TODO.md

@@ -79,6 +79,9 @@ order and what "done" looks like. Tick and date them in place.
     through, and the fixture recordings' expected velocity flipped sign
     with its magnitude unchanged.
 
+  **`docs/SCROLL.md` is the standing reference** for how scrolling works
+  now -- read that rather than reconstructing it from this entry.
+
   **Still open, and the one thing to decide:** the *pin* ("stay at the
   end as rows are appended") is still each widget's own -- `Scroll` has
   `snap_end` for an ordinary child, `LazySpan` has one for itself, and