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.

docs/SCRIPTING.md

@@ -168,6 +168,14 @@ spread within `r` of the object, each facing its rotation give or take 70°,
 each its own sprite. Flocks of birds, butterflies and bees use `swim` with a
 `shape`.
 
+## Groups and the drawing order
+
+Objects of type `group` are never passed to scripts (they draw nothing), nor is anything inside a
+hidden group (`Doc::hidden_in_groups`); prefabs never see the `group` field's effect. The order of
+`scene.json` is the drawing order of the `ground` layer (unsorted): later ground pieces lie on
+top. Sorted layers ignore it except for ties. The Studio keeps each group's members just before
+the group object (`tree`, `flatten` in studio.js) and sends tree edits as one `order` op.
+
 ## Routes
 
 `routes.rhai` (`route` prefab, meta `shape: "path"`: the Studio draws it point by point and