Let Spawn take an asset over an edge and find a child by name (#237)

An asset could only be picked as a setting on the Spawn node. It could
not be wired in, held in a variable or put into a list. Finding an
object below the own one needed a scene-wide search by name.

- "Asset" is now a kind like Animation or Sound: port, variable, list
  item and a new "Asset" literal node, with its own cube icon.
- Spawn and Is asset have an `asset` input. It uses the key of the old
  setting, so a saved asset reads as the value of the input and older
  projects keep spawning without a migration.
- New node "Child by name" searches below an object level by level.
  A child wins over a grandchild of the same name. Nothing found or no
  object gives null.

Tests:
- ./check.sh: 168 files, 2453 tests, CHECK EXIT 0.
- 9 new unit tests in tests/core/asset-kind.test.ts, catalog cases for
  Spawn, Is asset, Child by name and the literal.
- e2e 6z-ii (2 checks): node rows and a real drag of the literal onto
  `asset`. e2e 6: 119 checks passed. Spawn checks in 3 and 9 passed.
- Counter-checks: depth-first search fails 1, Spawn reading only the
  old setting fails 3, runtime ignoring the parent fails 1.

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

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

@@ -0,0 +1,20 @@
+import type { NodeDef } from '../../NodeRegistry';
+import { asEntity, asString } from '../../values';
+import { obj, str } from '../shared';
+
+export const ChildByName: NodeDef = {
+  type: 'ChildByName',
+  label: 'Child by name',
+  category: 'data',
+  description:
+    'Finds an object by its name below the given one – among its children, their children and so on. ' +
+    'The nearest level wins; within a level, the first in hierarchy order. Nothing when none is called so.',
+  inputs: [obj('object'), str('name')],
+  outputs: [obj('child')],
+  params: [],
+  run: (ctx, inputs) => {
+    const parent = asEntity(inputs.object);
+    const name = asString(inputs.name);
+    return { outputs: { child: parent ? ctx.childByName(parent, name) : null } };
+  },
+};

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

@@ -1,18 +1,17 @@
 import type { NodeDef } from '../../NodeRegistry';
-import { asEntity } from '../../values';
-import { assetParam, bool, obj } from '../shared';
+import { asEntity, asString } from '../../values';
+import { assetPort, bool, obj } from '../shared';
 
-import { textOf } from '../../../model/Properties';
 export const IsAsset: NodeDef = {
   type: 'IsAsset',
   label: 'Is asset',
   category: 'data',
   description: 'Whether an object is an instance of the chosen asset – the way to tell what you collided with.',
-  inputs: [obj('object')],
+  inputs: [obj('object'), assetPort()],
   outputs: [bool('result')],
-  params: [assetParam],
-  run: (_ctx, inputs, params) => {
-    const assetId = textOf(params.asset ?? '');
+  params: [],
+  run: (_ctx, inputs) => {
+    const assetId = asString(inputs.asset);
     return { outputs: { result: Boolean(assetId) && asEntity(inputs.object)?.assetId === assetId } };
   },
 };

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

@@ -50,6 +50,11 @@ export const AnimationLiteral = literal(
   'animation',
   'A constant animation, for a "Play animation", "Pause animation" or "Stop animation" to work on.',
 );
+export const AssetLiteral = literal(
+  'Asset',
+  'asset',
+  'A constant placeable asset, for a "Spawn" to place or an "Is asset" to compare with.',
+);
 export const ListLiteral = literal(
   'List',
   'list',