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/agent.js

@@ -262,6 +262,9 @@
   }
   function system() {
     var s = st(), S = s.S, sel = S.sel && s.obj(S.sel), v = s.view;
+    var many = (S.selIds || []).map(function (id) { return s.obj(id); }).filter(Boolean);
+    var selText = many.length > 1 ? "Selected " + many.length + " objects (\"these\", \"them\"): " + JSON.stringify(many.slice(0, 30).map(compact)) + (many.length > 30 ? " and " + (many.length - 30) + " more" : "") + "."
+      : sel ? "Selected (\"this\", \"it\"): " + JSON.stringify(compact(sel)) : "Nothing is selected.";
     return {
       role: "system",
       content: [
@@ -270,7 +273,7 @@
         "How to work: look before you build (list_objects shows what is where with world boxes; list_prefabs with a type gives its props). Place things inside the site, on free ground, and use the boxes to avoid overlaps. Put related edits into one apply_ops call so they undo as one step. After bigger changes, check with render_view or check_project. Use select_object to show the user what you mean. Ask back when a request is unclear instead of guessing at big changes.",
         "Answer in the language of the user's latest message, short and plain, without tables: say what you changed and give object ids in backticks (the user can click them). To show the result, write the answer first and call select_object in the same step: that ends your turn.",
         "The Studio right now: project \"" + S.meta.title + "\", " + (S.doc.objects || []).length + " objects; view level \"" + v.getLevel() + "\", rotation " + v.getRotation() + "; snap step " + (s.snapStep() || "off") + ". " +
-          (sel ? "Selected (\"this\", \"it\"): " + JSON.stringify(compact(sel)) : "Nothing is selected.") + " " + screenAxes(v.getRotation())
+          selText + " " + screenAxes(v.getRotation())
       ].filter(Boolean).join("\n\n")
     };
   }