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/editor/panels/icons/kind-asset.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="kind-asset" viewBox="-8 -8 16 16" width="16" height="16">
+  <path d="M0 -5.5 L5 -3 L5 3 L0 5.5 L-5 3 L-5 -3 Z" fill="none" stroke="currentColor" stroke-width="1.2" />
+  <path d="M-5 -3 L0 -0.5 L5 -3 M0 -0.5 L0 5.5" fill="none" stroke="currentColor" stroke-width="1.2" />
+</svg>

src/editor/panels/markIcons.ts

@@ -9,6 +9,7 @@ import arrowOut from './icons/arrow-out.svg?raw';
 import fitHeight from './icons/fit-height.svg?raw';
 import fitWidth from './icons/fit-width.svg?raw';
 import animation from './icons/kind-animation.svg?raw';
+import asset from './icons/kind-asset.svg?raw';
 import boolean from './icons/kind-boolean.svg?raw';
 import color from './icons/kind-color.svg?raw';
 import list from './icons/kind-list.svg?raw';
@@ -23,6 +24,7 @@ import reset from './icons/reset.svg?raw';
 /** Kinds shown as a picture; the rest keep their mark in letters (`#`, `Aa`). */
 export const KIND_ICON_SVG: Partial<Record<Kind, string>> = {
   animation,
+  asset,
   boolean,
   color,
   list,