Multi-select, select joined walls and fences, pond with swimming fish

Selection: S.sel stays the object picked last; S.selIds holds everything
selected.
- In the object list, Ctrl+click adds or removes an object and
  Shift+click selects a range.
- On the map, Ctrl/Shift+click toggles an object and Shift+drag draws a
  box that selects what lies wholly inside it (locked and hidden objects
  are left out).
- Ctrl+A selects everything that is not locked; Esc selects nothing.
- Clicking a wall or fence a second time selects every piece of the same
  type joined to it; another click goes back to that one piece.
- Dragging one selected object moves them all, together with the windows
  and doors on them, as one undo step. Arrow keys, R (turns the group
  about its middle), Ctrl+D, Delete, Lock and Hide all work on the whole
  selection.
- When all selected objects share a type, the property form edits them
  all; fields where they differ show "mixed".
- The agent's system prompt lists the selected objects.

The viewer outlines several objects with selectObj([ids]) and passes the
click event to onPick. A batch op now answers `ids`, one per operation,
so the copies from a group duplicate get selected.

Pond (Nature): natural, round or square, with a stone or grass edge,
fish, lily pads, spreading ripples and reed clumps. The fish come from a
new Swim primitive (script: s.swim). One matrix maps the object's frame
to the screen. Each fish turns round its ellipse with a CSS rotation,
so off-screen pausing and reduced motion apply as for the other
animations. The house demo gets a koi pond in the back garden.

Tests: ponds in every shape and rotation have the right number of fish,
no NaN and no depth-sort cycles, and stay inside their rectangle.
check_studio covers list and map multi-select, a group drag undone in
one step, joined-fence selection, a mixed-value form applied to all, box
select, delete and undo, Ctrl+A, Esc, and a new pond's fish.

studio/index.html

@@ -65,7 +65,8 @@ document.documentElement.setAttribute("data-theme",t);})();
     <tr><th>Draw</th><td>walls, fences, hedges, roads: drag along the map for one piece, or click corner after corner for a run (double-click or Enter ends it, clicking the first corner closes it, Backspace drops the last corner); ends snap onto the walls and fences they meet, which then join at corners and T-junctions. Floors, areas, buildings, roofs: drag a rectangle; Room: drag a rectangle for a floor with walls around it</td></tr>
     <tr><th>Windows, doors</th><td>click on a wall: they snap onto it, cut their opening and move, turn, copy and delete with the wall; R turns a door to the other hinge and swing side</td></tr>
     <tr><th>Storeys</th><td>GF, 1F, … cut a house open; on 1F and up, new walls, floors and furniture go onto that storey (3 m each, parameter <code>storey_h</code>)</td></tr>
-    <tr><th>Select</th><td>click it on the map or in the list</td></tr>
+    <tr><th>Select</th><td>click it on the map or in the list; click a wall or fence again to select every piece joined to it (again: just that one)</td></tr>
+    <tr><th>Several</th><td>Ctrl+click (map or list) adds or takes out one, Shift+click in the list selects a range, Shift+drag on the map selects what lies inside the box, Ctrl+A everything not locked, Esc none. Drag one of them, arrow keys, R, Ctrl+D and Delete then work on all; the form edits a property for all of them</td></tr>
     <tr><th>Move</th><td>drag it, or arrow keys (by the snap step)</td></tr>
     <tr><th>Turn</th><td>R / Shift+R, or the ⟲ ⟳ buttons</td></tr>
     <tr><th>Copy / delete</th><td>Ctrl+D / Delete</td></tr>