feat(ludic.lab): the visual lab as a package - a scene on a plate, never a world

A scene is an entry in the open registry LabScenes, shown on a plate: a
flat lit disc, the package's own small sky with the sun at one hour, and
a frame clock. Headless each camera (lab_shot, lab_shot_at) settles and
is written as build/lab/<scene>/<shot>.png (a stored PNG, written here);
in a window N and P step through them. tools/lab/run.sh and sheet.py
make the contact sheet; tools/lab/plate_build.py makes plate/.
ludic.render3d gains r3d_plate_mode (no terrain, grass or water is made)
and r3d_sky_path. quit() in a program with no frame loop links. The
example takes ~120 MB resident, 417 MiB peak footprint. Reseed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.lab/plate/plate.gltf

@@ -0,0 +1,129 @@
+{
+ "asset": {
+  "version": "2.0",
+  "generator": "ludic tools/lab/plate_build.py"
+ },
+ "scene": 0,
+ "scenes": [
+  {
+   "nodes": [
+    0
+   ]
+  }
+ ],
+ "nodes": [
+  {
+   "name": "plate",
+   "mesh": 0
+  }
+ ],
+ "meshes": [
+  {
+   "name": "plate",
+   "primitives": [
+    {
+     "attributes": {
+      "POSITION": 0,
+      "NORMAL": 1,
+      "TEXCOORD_0": 2
+     },
+     "indices": 3,
+     "material": 0
+    }
+   ]
+  }
+ ],
+ "materials": [
+  {
+   "name": "plate",
+   "pbrMetallicRoughness": {
+    "baseColorTexture": {
+     "index": 0
+    },
+    "metallicRoughnessTexture": {
+     "index": 1
+    }
+   }
+  }
+ ],
+ "textures": [
+  {
+   "source": 0
+  },
+  {
+   "source": 1
+  }
+ ],
+ "images": [
+  {
+   "uri": "plate_diff.png"
+  },
+  {
+   "uri": "plate_arm.png"
+  }
+ ],
+ "buffers": [
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+   "uri": "plate.bin",
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+  }
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+}