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/runtime/World.ts

@@ -410,6 +410,19 @@ export class World {
     return this.entities.find((e) => e.name === name);
   }
 
+  /** The nearest entity with that name below `parent`, level by level – a child before a grandchild. */
+  childByName(parent: Entity, name: string): Entity | undefined {
+    for (
+      let level = this.childrenOf(parent);
+      level.length > 0;
+      level = level.flatMap((e) => this.childrenOf(e))
+    ) {
+      const found = level.find((e) => e.name === name);
+      if (found) return found;
+    }
+    return undefined;
+  }
+
   /**
    * The camera the game is seen through: of the cameras that are switched on (itself and every
    * parent, see `properties/enabled.ts`) the first one in hierarchy order. Undefined while none

src/runtime/systems/BehaviorSystem.ts

@@ -662,6 +662,10 @@ export class BehaviorSystem implements System {
       time: world.time,
       dt,
       findByName: (name) => world.findByName(name) ?? null,
+      childByName: (parent, name) => {
+        const found = resolve(parent);
+        return (found && world.childByName(found, name)) ?? null;
+      },
       isKeyDown: (key) => this.input.isDown(key),
       pointer: world.pointer,
       hitTest: (target, x, y) => {