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/shared.ts

@@ -49,6 +49,12 @@ export const keyParam: PropertyDef = { key: 'key', label: 'Key', kind: 'key', de
 /** No `assetTypes`: any placeable asset is a valid answer here. */
 export const assetParam: PropertyDef = { key: 'asset', label: 'Asset', kind: 'asset', default: '' };
 
+/**
+ * The `asset` input of `Spawn` and `Is asset`; unconnected, it shows the asset picker. The key is the
+ * one the setting had before it became a port, so a saved pick is the inline value now.
+ */
+export const assetPort = (): PortDef => ({ key: 'asset', label: 'asset', type: 'string', field: assetParam });
+
 /**
  * The `sound` input of the two sound nodes; unconnected, it shows the sound picker – the way
  * `machinePort` shows the machine picker. A `Sound` literal is what feeds it through an edge.