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/studio.css

@@ -159,6 +159,10 @@ input[type=checkbox] { accent-color: var(--accent); }
 .ag-grip { position: absolute; left: 0; top: 0; width: 14px; height: 14px; cursor: nwse-resize; touch-action: none; background: linear-gradient(135deg, var(--mute) 0 2px, transparent 2px 4px, var(--mute) 4px 5px, transparent 5px); opacity: .5; border-top-left-radius: 8px; }
 .ag-grip:hover { opacity: 1; }
 
+/* the selection box (Shift / Ctrl + drag on the map) */
+.st-band { position: fixed; z-index: 30; pointer-events: none; border: 1px dashed var(--accent); background: color-mix(in srgb, var(--accent) 12%, transparent); }
+#objects { user-select: none; }
+
 /* previews while placing: footprints, wall planes, where a window lands */
 .iso-svg .st-ghosts { pointer-events: none; }
 .iso-svg .st-ghost { fill: color-mix(in srgb, var(--iso-accent) 16%, transparent); stroke: var(--iso-accent); stroke-width: 1.5; stroke-dasharray: 5 3; vector-effect: non-scaling-stroke; stroke-linejoin: round; }