Describe folding object groups and summary keys in the timeline (#414)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Animations.md

@@ -130,7 +130,15 @@ frames about the pointer, from 48 pixels a frame out to 50 frames a pixel, so an
 fits a screen (#407). `Shift`+wheel scrolls across, `Shift`+drag pans across, the middle button
 pans both ways. **More rows than room** (#413): the wheel over the header column scrolls the rows
 up and down, the ruler stays where it is, and a scrollbar at the right edge can be dragged. A row
-picked or added out of view is scrolled into it. Groups cannot be collapsed yet.
+picked or added out of view is scrolled into it. **Folding a group** (#414): a click on an
+object's head, or the triangle in front of its name, folds its rows away, and the groups of the
+objects below it fold with it. The root's head stands level with its children, so folding it hides
+only its own rows. Every head shows **summary keys**, folded or not: a diamond on each frame on
+which any row of the group, or of a group below it, has a key. A summary key stands for all of those
+keys: a click picks them, a drag moves them (`Ctrl`+drag duplicates), a band catches them, `Delete`
+removes them, each one undo step. Values stay with the rows, as they differ from row to row. Folding
+is a way of looking: it is no undo step and lasts for the session, per animation. A row picked on
+its own while its group is folded, one just added for example, opens the group.
 Scrubbing while the animation plays pauses it, and the button turns back to *Play*.
 
 **Play, stop, pause.** `Play animation` starts an animation on an object (both are ports: the