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.

viewer/isokit-view.css

@@ -108,6 +108,9 @@ html[data-theme=light] .iso-svg .iso-glint { display: inline; }
 .iso-svg .iso-shuttle { animation: iso-shuttle 48s ease-in-out infinite; }
 .iso-svg .iso-drive { animation: iso-drive 16s linear infinite; }
 .iso-svg .iso-ledr, .iso-svg .iso-ledo { animation: iso-blink 2.4s ease-in-out infinite; }
+/* fish: each turns round its pond's middle (the box is centred there), at its own speed */
+.iso-svg .iso-swim { transform-box: fill-box; transform-origin: center; animation: iso-spin 20s linear infinite; }
+.iso-svg .iso-rip { transform-box: fill-box; transform-origin: center; animation: iso-rip 5.1s ease-out infinite; }
 .iso-svg .iso-park { display: none; }
 .iso-off .iso-a, .iso-still .iso-a, .iso-off .iso-drive, .iso-still .iso-drive { animation-play-state: paused !important; }
 .iso-still .iso-park { display: inline; }
@@ -117,6 +120,7 @@ html[data-theme=light] .iso-svg .iso-glint { display: inline; }
   12% { opacity: .75; }
   100% { transform: translate(5px, -28px) scale(2); opacity: 0; }
 }
+@keyframes iso-rip { 0% { transform: scale(.2); opacity: 0; } 15% { opacity: .9; } 100% { transform: scale(1.5); opacity: 0; } }
 @keyframes iso-glint { 0%, 84%, 100% { opacity: 0; } 91% { opacity: .9; } }
 @keyframes iso-blink { 0%, 100% { opacity: 1; } 50% { opacity: .45; } }
 @keyframes iso-move { from { transform: translate(0, 0); } to { transform: translate(var(--dx), var(--dy)); } }
@@ -133,7 +137,7 @@ html[data-theme=light] .iso-svg .iso-glint { display: inline; }
 }
 @media (prefers-reduced-motion: reduce) {
   .iso-svg .iso-a { animation: none !important; }
-  .iso-svg .iso-puff, .iso-svg .iso-puff2, .iso-svg .iso-smoke, .iso-svg .iso-disc, .iso-svg .iso-glint { display: none; }
+  .iso-svg .iso-puff, .iso-svg .iso-puff2, .iso-svg .iso-smoke, .iso-svg .iso-disc, .iso-svg .iso-glint, .iso-svg .iso-rip { display: none; }
   .iso-svg .iso-park { display: inline; }
 }
 @media (max-width: 640px) {