Pick the target of an object variable in the Scene View (#276)

Choosing the target of an object variable meant finding its name in
a dropdown. With many objects, or several with the same sprite, the
name alone does not say which one it is. The eyedropper lets you
click the object where you see it instead.

- Every object dropdown gets an eyedropper button next to it. This
  covers variables, struct fields and list entries, in the Inspector
  and in the Variables tab.
- The button opens the Scene View or the container tab the object
  lives in. A strip above the view names the field and the current
  candidate and offers Take and Cancel. The cursor is a crosshair.
- A click picks a candidate and writes nothing yet. The candidate
  gets its own cyan frame, the selection stays on the object with
  the variable.
- Clicking follows the selection rule. `pickOn` in
  `src/viewport/pick.ts` uses `pressOn` and only counts objects the
  dropdown offers. A click on empty space keeps the candidate.
- Enter or Take writes through the same handlers as the dropdown.
  That is one undo step, or none if nothing changes.
- Esc, Cancel, a right click in the view or a second click on the
  button cancel without writing. So do a tab switch, another
  selection, undo, redo, play and loading a project.
- After Take or Cancel the editor goes back to the tab the pick came
  from.
- While the eyedropper runs, the view does not drag, marquee, paint,
  show handles or open its context menu. Panning and zooming stay.

Found on the way:

- Started from the Variables tab, the click missed the object. The
  Scene View is hidden while that tab is open, and the layout skips
  hidden children. The strip appeared then, the view moved down by
  32 px but kept its old height. `Editor.display()` now lays the row
  out again after showing it.

Tests (on melektaus-atx):

- unit pick and object-pick: 27 passed, 13 new
- gate: 196 files, 2844 tests, CHECK EXIT 0. Two more come from
  svg-icons, which checks every icon twice.
- e2e 57: ALL PASSED, 12 checks, 5 new (57e to 57g)
- Counter-checks work. Without the layout fix 57g fails. Picking
  objects the dropdown does not offer fails 2 tests. Writing on
  cancel fails 2 more.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -45,7 +45,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1591 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1596 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs