Let a graph read and change the z of an object

Until now a behavior could not move an object in front of another
while the game runs. Set position writes only x and y, and z lives on
the running object rather than in its props, so Properties cannot
reach it either. Drawing already reads the z every frame, so only the
nodes were missing.

- Get Z (data): object in, z and world z out. World z is summed up
  with the parents, the way drawing does it (worldTransformOf).
- Set Z (action): object and z in. It sets the object's own z, local
  to its parent like Set position. Higher is drawn in front from the
  next frame on.
- Two nodes of their own rather than a z port on Set position and
  Get position, as asked.

Tests:
- node-catalog: Get Z for a child (2 below 5 is world z 7) and for no
  object, Set Z writes only z and leaves x/y.
- set-z.test.ts: in a running world a key sets the lower of two
  sprites to z 2, drawSlots then draws it in front, Get Z logs 2 and 2.
- Two counter-checks bite: Set Z without the write and Get Z without
  the parent fail 3 tests.
- Gate: 233 files, 3548 tests (3542 + 6), CHECK EXIT 0.

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

src/core/graph/nodes/data/GetZ.ts

@@ -0,0 +1,19 @@
+import { worldTransformOf } from '../../NodeContext';
+import type { NodeDef } from '../../NodeRegistry';
+import { asEntity } from '../../values';
+import { num, obj } from '../shared';
+
+export const GetZ: NodeDef = {
+  type: 'GetZ',
+  label: 'Get Z',
+  category: 'data',
+  description:
+    'The z of an object, which decides what is drawn in front: local to its parent (z) and summed up (world z).',
+  inputs: [obj('object')],
+  outputs: [num('z'), num('worldZ', 'world z')],
+  params: [],
+  run: (_ctx, inputs) => {
+    const target = asEntity(inputs.object);
+    return { outputs: { z: target?.z ?? 0, worldZ: target ? worldTransformOf(target).z : 0 } };
+  },
+};