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>

tests/core/node-catalog.test.ts

@@ -423,6 +423,23 @@ const CASES: Record<string, Case[]> = {
     },
     { name: 'an unknown name is null', inputs: { name: 'Ghost' }, outputs: { object: null } },
   ],
+  ChildByName: [
+    {
+      name: 'asks the context below the object it is given',
+      inputs: { object: fakeEntity('Player'), name: 'Hitbox' },
+      context: {
+        childByName: (parent, name) =>
+          parent.name === 'Player' && name === 'Hitbox' ? fakeEntity('Hitbox') : null,
+      },
+      outputs: { child: fakeEntity('Hitbox') },
+    },
+    {
+      name: 'an unknown name is null',
+      inputs: { object: fakeEntity('Player'), name: 'Ghost' },
+      outputs: { child: null },
+    },
+    { name: 'no object is null', inputs: { object: null, name: 'Hitbox' }, outputs: { child: null } },
+  ],
   NameOf: [
     { name: 'reads the name', inputs: { object: fakeEntity('Coin') }, outputs: { name: 'Coin' } },
     { name: 'no object is the empty name', inputs: { object: null }, outputs: { name: '' } },
@@ -430,10 +447,14 @@ const CASES: Record<string, Case[]> = {
   IsAsset: [
     {
       name: 'matches the asset id',
-      inputs: { object: fakeEntity('Coin', 0, 'coin') },
-      params: { asset: 'coin' },
+      inputs: { object: fakeEntity('Coin', 0, 'coin'), asset: 'coin' },
       outputs: { result: true },
     },
+    {
+      name: 'another asset does not match',
+      inputs: { object: fakeEntity('Coin', 0, 'coin'), asset: 'gem' },
+      outputs: { result: false },
+    },
     {
       name: 'no asset chosen never matches',
       inputs: { object: fakeEntity('Coin', 0, 'coin') },
@@ -504,6 +525,10 @@ const CASES: Record<string, Case[]> = {
     { name: 'outputs the id it was set to', params: { value: 'anim-1' }, outputs: { value: 'anim-1' } },
     { name: 'unset is no animation', outputs: { value: '' } },
   ],
+  Asset: [
+    { name: 'outputs the id it was set to', params: { value: 'crate' }, outputs: { value: 'crate' } },
+    { name: 'unset is no asset', outputs: { value: '' } },
+  ],
   Add: [
     { name: 'adds', inputs: { a: 2, b: 3 }, outputs: { result: 5 } },
     { name: 'numeric text is coerced', inputs: { a: '2', b: 3 }, outputs: { result: 5 } },
@@ -1322,6 +1347,22 @@ const CASES: Record<string, Case[]> = {
   ],
   Tile: [{ name: 'hands out the tile chosen', params: { value: 'door' }, outputs: { value: 'door' } }],
   Spawn: [
+    {
+      name: 'places the asset that arrives on its input, local to the parent',
+      inputs: { asset: 'crate', x: 1, y: 2, z: 3, parent: fakeEntity('Box') },
+      context: {
+        spawn: (assetId, position, parent) =>
+          assetId === 'crate' &&
+          position.x === 1 &&
+          position.y === 2 &&
+          position.z === 3 &&
+          parent?.name === 'Box'
+            ? fakeEntity('Crate')
+            : null,
+      },
+      next: 'exec',
+      outputs: { object: fakeEntity('Crate') },
+    },
     {
       name: 'no asset chosen logs a hint and continues with no object',
       next: 'exec',