Make Iris layout dependencies explicit

docs/SCROLL.md

@@ -1,6 +1,6 @@
 # Scrolling in iris
 
-How anything in iris scrolls, as of 2026-09-08. This is the current
+How anything in iris scrolls, as of 2026-09-09. This is the current
 design, not a history — the git log has the account of
 how it got here, and `docs/IRIS_TODO.md` has what is still open.
 
@@ -247,6 +247,23 @@ drawn (Iris, 2026-09-08). A correction that lands next frame is a frame
 drawn wrong, and there may be no next frame — a fling that stopped is not
 asking for one.
 
+## Directional input
+
+A scrollable registers `CursorSense::drag(axis)` and
+`CursorSense::Scroll(axis)`. Horizontal and vertical gestures are distinct
+input semantics, so a higher horizontal row does not consume an undecided
+press that may belong to the lower vertical transcript. Both may observe the
+press start; after movement crosses `DRAG_SLOP`, the pointer locks to its
+dominant axis, only the matching listener receives the drag, and capture
+cancels every other listener that had been tracking the press.
+
+Visual layers still decide priority between listeners for the same semantic.
+`drag_senses()` remains the deliberately direction-agnostic form for widgets
+such as selection that arbitrate the gesture themselves. Wheel input follows
+the same axis split; the desktop backend's Shift+wheel mapping produces a
+horizontal delta before dispatch, so it reaches the horizontal listener
+without a scroll-widget special case.
+
 ## The transcript's wiring
 
 `app-rust/src/ui/mod.rs`, `build_tree`.