Object tree: groups and the drawing order

Groups: objects of type "group" draw nothing; members carry the group's id
in a new `group` field (groups nest). A hidden group hides everything in it
(Doc::hidden_in_groups, skipped by the driver), a locked one locks it in
the Studio. Removing a group moves its members up into its parent group.
isokit check warns about members of missing groups. New `order` op: the
whole object order at once (each id once), one undo step.

Studio object list as a tree in the document order, front first: each
group sits just after its members, so it draws as one block, and ground
pieces higher in the list lie on top. Groups fold open and shut; every row
has an eye and a lock (shown on hover or when set); a click on a group
selects all of it, so a drag on the map moves it all. Group / Ungroup
(Ctrl+G, Ctrl+Shift+G), Order up/down (Ctrl+] Ctrl+[, with Shift to the
front / back), drag rows above, below or into a group. Group panel: name,
zoom, ungroup, to front / back, lock, hide, delete with contents.

MCP: group field in the object schema, order op, guidance.

Tests: Rust 81 (ops: groups, order, removal; driver: nested hidden
groups), Studio 131, web 100.

README.md

@@ -48,9 +48,11 @@ Try the demos: `isokit studio examples/demo` (a works site),
 | Do | How |
 |---|---|
 | Add | pick an object on the left, click on the map (Shift-click places several) |
-| Draw | walls, fences, hedges, roads: drag along the map, or click corner after corner for a run around a plot (double-click / Enter ends it, the first corner closes it); ends snap onto the walls and fences they meet; a road run gets a curve in each corner. Pieces keep to x or y, or go diagonal at 45° once the pointer is more than 22.5° off; hold Ctrl for any angle Floors, areas, buildings, roofs, rugs: drag a rectangle |
+| Draw | walls, fences, hedges, roads: drag along the map, or click corner after corner for a run around a plot (double-click / Enter ends it, the first corner closes it); ends snap onto the walls and fences they meet; a road run gets a curve in each corner. Pieces keep to x or y, or go diagonal at 45° once the pointer is more than 22.5° off; hold Ctrl for any angle. Floors, areas, buildings, roofs, rugs: drag a rectangle. Routes: click point after point |
 | Select | click it on the map or in the object list; click a wall or fence again to select every piece joined to it |
 | Several | Ctrl+click (map or list) adds or takes out one, Shift+click in the list a range, Shift+drag on the map a box, Ctrl+A all; then drag one to move them all, arrows, R, Ctrl+D, Delete and the property form work on all of them |
+| Group | select several, then ▤ Group or Ctrl+G; Ctrl+Shift+G ungroups. In the object list a group folds open and shut, a click on it selects all of it (drag one on the map to move them all), its eye and lock hide or lock all of it; drag rows into or out of a group |
+| Order | the object list is the drawing order of ground pieces (areas, beds, ponds, roads): higher lies on top. Drag rows, or ▲ ▼ / Ctrl+] Ctrl+[ (with Shift: to the front / back) |
 | Move | drag it, or arrow keys by the snap step (0.1 / 0.5 / 1 / 5 m) |
 | Turn | R / Shift+R or ⟲ ⟳ (quarter turns), or type any angle in Rotation |
 | Edit | the right panel: size, storeys, colours, solar, tooltip text, label, levels |