Clip the children of an object to a clip area (#260)

An inventory grid could not scroll, because nothing could cut a child
off at an edge. A game also had no way to hear the mouse wheel. Both
are needed for a scrolling list, a dialog text or a window with more
content than room.

- A new object type Clip area has width and height and turns, scales
  and mirrors with its parent. The size gizmo drags width and height.
- Everything below a clip area, on every level, is drawn only inside
  its rectangle, in the Scene view as in the game. Nested areas cut to
  their intersection. The rule works on a convex polygon, so a turned
  area cuts right as well.
- On click, Is hovered and On mouse wheel do not hit a cut-off part.
  The Scene view does not pick it either.
- In the Scene view the area is a dashed frame that is grabbed only at
  its edge, so a click inside lands on the content. The eye under
  Shape hides the frame. In the game the area is invisible.
- While an object below an area or one of its parents is selected, the
  Scene view shows the cut-off part at 30 % opacity and it can be
  grabbed there. The rule lives in core/properties/clip.ts.
- The Properties node knows the type Clip area.
- The new event node On mouse wheel fires by the rule of On click. Its
  delta counts in notches, positive is down, and a frame's turns
  arrive as one value. It also gives x and y.

Limits:

- Only sprites and texts show the faded rest. Tilemaps and particles
  are cut and can be grabbed when open, but are not shown faded.
- Masks by texture shape are out, as noted in the issue.

Tests:

- ./check.sh: 2773 tests, CHECK EXIT 0.
- Full e2e run (local): ALL PASSED, 1549 checks, measured.
- tests/core/clip-areas.test.ts: 12 tests for the cut rule, scene and
  game, save and load, hits, the wheel with and without owner, the
  notch conversion and the open rule.
- e2e 58, 5 checks with real mouse and wheel: the scene cuts (74 of 74
  red inside, 0 outside), a click on the cut part picks nothing, the
  selected rest is faded and grabbable, the game cuts, one notch
  logs 1.
- e2e 04 and 46 count the object types and the eye rows. They now
  include Clip area.
- Counter-checks: 6 in unit tests and 3 in the browser, all fail as
  expected. The originals were restored and checked with cmp.

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

tests/helpers.ts

@@ -185,6 +185,7 @@ export const NO_POINTER: PointerState = {
   down: false,
   pressed: false,
   released: false,
+  wheel: 0,
 };
 
 /** A top-level entity handle named after itself, with empty props. */