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>

AGENTS.md

@@ -76,6 +76,11 @@ Module-by-module intent is in `docs/PLAN.md`'s "Backend layout".
     public API** -- Iris, 2026-09-08), working list, decisions log,
     layout/render design, and texture-atlas design, and the client-core
     crate's design, respectively.
+  - `docs/SCROLL.md` — how anything in iris scrolls: `Scroll` owns the
+    position, the gesture and the fling; a child either gets moved or
+    answers `Widget::scrolls_itself` and is handed deltas. Read it before
+    touching `scroll.rs`, `lazy_span.rs`, or anything that pans, flings
+    or lays out a long list.
 - `.dev-updater.ron` — what Dev Updater builds here: the server (run as
   `service: Managed(…)`, supervised by Dev Updater's own implementation
   rather than a script kept here) and the APK, in parallel. It points at