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.

examples/house/scene.json

@@ -7,6 +7,7 @@
     {"id": "path", "type": "area", "x": -1.0, "y": -6.05, "props": {"w": 1.4, "d": 2.9, "mat": "g-path"}, "name": "Front path"},
     {"id": "terrace", "type": "area", "x": 8.6, "y": -2.25, "props": {"w": 5.0, "d": 4.5, "mat": "g-pad"}, "name": "Terrace"},
     {"id": "garden", "type": "area", "x": 2, "y": 9.2, "props": {"w": 30, "d": 7.5, "mat": "g-garden"}, "name": "Back garden"},
+    {"id": "pond", "type": "pond", "name": "Fish pond", "x": -4.5, "y": 9.4, "props": {"w": 6.5, "d": 3.8, "fish": 6, "lilies": 4}, "info": "Koi pond, 6.5 x 3.8 m"},
     {"id": "floor-living", "type": "floor", "x": -3.0, "y": -2.0, "props": {"w": 6, "d": 5, "mat": "f-oak"}, "name": "Living room"},
     {"id": "floor-kitchen", "type": "floor", "x": 3.0, "y": -2.0, "props": {"w": 6, "d": 5, "mat": "f-tile"}, "name": "Kitchen and dining"},
     {"id": "floor-bedroom", "type": "floor", "x": -3.5, "y": 2.5, "props": {"w": 5, "d": 4, "mat": "f-carpet"}, "name": "Bedroom"},