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.

crates/isokit-script/src/api.rs

@@ -432,6 +432,32 @@ pub fn register_sprite(e: &mut Engine) {
             ok(s)
         },
     );
+    // fish round ellipses: list of #{ rx, ry, dur, delay, cw, len, cls }
+    e.register_fn(
+        "swim",
+        |s: &mut SpriteH, x: D, y: D, z: D, list: Array| -> RR<SpriteH> {
+            let mut fish = Vec::with_capacity(list.len());
+            for f in &list {
+                let Some(m) = f.read_lock::<Map>() else {
+                    return err("swim: expected a list of maps");
+                };
+                fish.push(isokit_core::Fish {
+                    rx: fnum(&m, "rx"),
+                    ry: fnum(&m, "ry"),
+                    dur: fnum(&m, "dur"),
+                    delay: fnum(&m, "delay"),
+                    cw: m.get("cw").and_then(|v| v.as_bool().ok()).unwrap_or(false),
+                    len: m.get("len").and_then(|v| n(v).ok()).unwrap_or(0.4),
+                    cls: m
+                        .get("cls")
+                        .map(|v| v.to_string())
+                        .unwrap_or_else(|| "koi".into()),
+                });
+            }
+            s.0.borrow_mut().swim(n(&x)?, n(&y)?, n(&z)?, fish);
+            ok(s)
+        },
+    );
     e.register_fn("raw", |s: &mut SpriteH, svg: &str| -> RR<SpriteH> {
         s.0.borrow_mut().raw(None, svg);
         ok(s)