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.

README.md

@@ -47,7 +47,8 @@ Try the demos: `isokit studio examples/demo` (a works site) and
 |---|---|
 | 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. Floors, areas, buildings, roofs, rugs: drag a rectangle |
-| Select | click it on the map or in the object list |
+| 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 |
 | Move | drag it, or arrow keys by the snap step (0.1 / 0.5 / 1 / 5 m) |
 | Turn | R / Shift+R or ⟲ ⟳ |
 | Edit | the right panel: size, storeys, colours, solar, tooltip text, label, levels |
@@ -72,6 +73,10 @@ The **House** palette has the parts, **Furniture** the interior:
 4. **Roof** (hip, gable, flat, optional solar) as a rectangle over the walls,
    and **Stairs**.
 
+**Garden**: the Pond (natural, round or square; stone or grass edge) has fish
+swimming round under lily pads, ripples and reeds; trees, hedges and rows of
+trees are in Nature.
+
 **Fences and walls** around the plot: the Fence has six designs (mesh, wooden
 boards, steel bars, mesh on a stone base, cobble wall, brick wall with
 pillars). Fences and site walls whose ends meet join like house walls: corners