Guides: drag guide rows to another layer; put older guides on a layer

Guide rows in the Layers panel drag (the selected guides when the row is
one of them) onto any row of another layer, which lights up, and go to
that layer in one undo step (guide.setLayer takes indexes). Guides on no
layer, from files saved before guides had layers, go on the top visible,
unlocked layer when a document opens or is reverted.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01CC5DiGJEZ5ADaQMBEQk4W1

docs/control-protocol.md

@@ -257,9 +257,10 @@ where it is greyed. The modifier keys still work while dragging (Cmd on a corner
 perspective, Cmd on a side shears).
 
 Ruler guides are layer objects: `guide.add` puts a guide on the current layer (or `layer`), where it shows, hides,
-locks and is deleted with its layer and is listed in the Layers panel (a click on its row selects it);
-`guide.setLayer {index?, layer}` moves guides to another layer, and `guide.list` gives each
-guide's `layer`, `shown` and `editable`.
+locks and is deleted with its layer and is listed in the Layers panel (a click on its row selects it, a drag onto
+another layer's rows moves it there); `guide.setLayer {index?, indexes?, layer}` moves guides to another layer, and
+`guide.list` gives each guide's `layer`, `shown` and `editable`. Guides in files saved before guides had layers go on
+the top visible, unlocked layer when the document opens.
 
 Floating panels: dragging a dock tab, a panel icon or a popped-out panel's title out of the dock floats that panel
 inside the window, and the strip right of the dock's tabs floats the whole Properties | Layers | Libraries group. A