Scroll sideways with Shift or Alt and the wheel, and in the outline

A wheel that tilts (or a touchpad) already panned the canvas sideways,
but only when the terminal passes those events on, and many don't:
Windows Terminal only does since 1.24, and Windows' own console layer
can drop them. The wheel with Shift held now scrolls sideways too, as in
most apps, and so does Alt, since Alacritty and Windows Terminal keep
Shift with the mouse for selecting text.

The outline scrolls sideways as well (the wheel, zh and zl), for rows
wider than its pane: a deep tree or a long title. Clicks land where the
rows are, however far they're scrolled, and when the cursor moves its
title comes back into view.

Fixed on the way: the wheel turned sideways over the help, the menu, a
picker or the K card moved it up.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

@@ -50,7 +50,8 @@ The plan for the everyday keys (all of them are in DESIGN.md):
 
 After the first key of a sequence (`g`, `z`, `d`, `<Space>`…), a popup lists
 what can follow. The right mouse button opens a menu of what you can do with
-a node.
+a node. The wheel scrolls; with Shift or Alt held (or a wheel that tilts), it
+scrolls sideways.
 
 ## Development