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.

studio/index.html

@@ -38,6 +38,13 @@ document.documentElement.setAttribute("data-theme",t);})();
     <section class="st-panel st-grow-panel">
       <h2>Objects <small id="ocount"></small></h2>
       <input type="search" id="osearch" placeholder="Filter…" autocomplete="off">
+      <div class="st-otools">
+        <button type="button" id="ogroup" title="Group the selected objects (Ctrl+G)">▤ Group</button>
+        <button type="button" id="oungroup" title="Ungroup (Ctrl+Shift+G)">Ungroup</button>
+        <span title="The order draws ground pieces (areas, beds, ponds, roads): higher in the list lies on top. Drag rows to reorder, or into a group">Order</span>
+        <button type="button" id="ofwd" title="Forward: draw on top of the one above (Ctrl+]); Shift: to the front (Ctrl+Shift+])">▲</button>
+        <button type="button" id="obwd" title="Backward (Ctrl+[); Shift: to the back (Ctrl+Shift+[)">▼</button>
+      </div>
       <ul id="objects" class="st-objects"></ul>
     </section>
   </aside>