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/ROADMAP.md

@@ -83,7 +83,10 @@ Each phase ends with tests, a Studio/viewer check in Edge, commit and push.
 
 Done since: the middle mouse button only moves the camera; fences and site walls join like house
 walls (`line_joint`); fence designs (mesh, wood, bars, half stone, cobble, brick with pillars);
-runs of walls and fences drawn corner by corner, ends snapping onto the pieces they meet.
+runs of walls and fences drawn corner by corner, ends snapping onto the pieces they meet;
+multi-select (Ctrl+click on the map and in the list, Shift range, Shift+drag box, Ctrl+A; group drag,
+nudge, turn, copy, delete and property edits; a second click on a wall or fence selects the joined
+run); the Pond with fish swimming round (`swim` primitive, CSS rotation), lily pads, ripples, reeds.
 
 ## Phase 2: richer editing