Fork the kit into contemporary-props with a nineteen slot palette, a per-model check that needs no registry round trip, and the first model.

contemporary-props/models/core/parts.ts

@@ -0,0 +1,49 @@
+import { Group } from 'three'
+
+import type { PartHandle } from '@vibe3d/model.ts'
+
+/**
+ * Semantic part slot: fixed anchor + replaceable content.
+ *
+ * This is the most easily misunderstood contract in vibe3d. `PartHandle`
+ * declares two separate objects and they CANNOT BE THE SAME:
+ *
+ *   anchor   the same object for the lifetime of the model. The consumer
+ *            attaches its light, its label, its collision body, its gameplay
+ *            object here.
+ *   content  the geometry that is thrown away and rebuilt every time
+ *            configure() is called.
+ *
+ * Making both the same Group and calling `clear()` on rebuild silently deletes
+ * everything the consumer attached too — the model keeps working, but the
+ * protocol's actual promise is broken. `scripts/verify-model.ts` tests this
+ * separately.
+ */
+export interface PartSlot extends PartHandle<Group> {
+  readonly anchor: Group
+  readonly content: Group
+  /** Replaces the content with a fresh Group and returns it. Leaves the anchor alone. */
+  reset(): Group
+}
+
+export function createPart(name: string): PartSlot {
+  const anchor = new Group()
+  anchor.name = name
+  let content = new Group()
+  content.name = `${name}/content`
+  anchor.add(content)
+
+  return {
+    anchor,
+    get content(): Group {
+      return content
+    },
+    reset(): Group {
+      anchor.remove(content)
+      content = new Group()
+      content.name = `${name}/content`
+      anchor.add(content)
+      return content
+    },
+  }
+}