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/index.ts

@@ -38,6 +38,7 @@ import { FindText } from './data/FindText';
 import { FirstNotEmpty } from './data/FirstNotEmpty';
 import { Floor } from './data/Floor';
 import { GetPosition } from './data/GetPosition';
+import { ChildByName } from './data/ChildByName';
 import { IsAsset } from './data/IsAsset';
 import { IsEnabled } from './data/IsEnabled';
 import { IsHovered } from './data/IsHovered';
@@ -48,6 +49,7 @@ import { KeyIsDown } from './data/KeyIsDown';
 import { Length } from './data/Length';
 import {
   AnimationLiteral,
+  AssetLiteral,
   BooleanLiteral,
   ListLiteral,
   NumberLiteral,
@@ -160,6 +162,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   Owner,
   Parent,
   ObjectByName,
+  ChildByName,
   NameOf,
   IsAsset,
   IsEnabled,
@@ -173,6 +176,7 @@ export const BUILT_IN_NODES: readonly NodeDef[] = [
   SoundLiteral,
   SceneLiteral,
   AnimationLiteral,
+  AssetLiteral,
   Add,
   Subtract,
   Multiply,