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.

docs/SCRIPTING.md

@@ -87,6 +87,7 @@ shaded automatically by face direction.
 | `circle(x, y, z, r_px, cls[, dx, dy])`, `stem(x, y, z0, z1, w_px, cls)` | screen-space details (crowns, poles) |
 | `person(x, y, z[, cls])`, `sit(x, y, z, cls)`, `head(x, y, z, r)`, `led(x, y, z[, cls])` | figures |
 | `puffs(x, y, z[, kind[, n, dur]])`, `fan(x, y, z, r)` | animated steam / smoke / fans |
+| `swim(x, y, z, [#{ rx, ry, dur, delay, cw, len, cls }, ...])` | fish going round ellipses (radii rx, ry along the object's axes) about (x, y) on the plane at z, one round per `dur` s, clockwise when `cw`; `cls` koi / koiw / koig (the pond) |
 | `seq([[x, y, \|q\| …], …])` | items drawn back to front by their anchor |
 | `group(cls[, css], \|q\| …)`, `begin(cls[, css])` … `end()` | `<g>` around parts (animation) |
 | `ext(x, y, z)`, `ext_box(…)` | widen the sort box |