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/scenes.ludic

@@ -0,0 +1,20 @@
+# scenes.ludic - every scene the lab can show, added by whoever has something to look at:
+#   def LabScenes deer { setup: fn deer_setup, tick: fn deer_tick }
+# setup builds what the scene shows and names its cameras (lab_shot); tick, if any, moves it each
+# frame by the lab's fixed step; draw, if any, draws what is not an actor or a scatter layer.
+export property LabScene {
+  key: string = ""
+  setup: fn() -> void = null
+  tick: fn(float) -> void = null
+  draw: fn() -> void = null
+}
+export open registry LabScenes of LabScene as LAB
+
+export function lab_scene_names() -> string {
+  var s = ""
+  for i in 0 .. LAB_COUNT {
+    if i > 0 { s = s + ", " }
+    s = s + LabScenes[i].key
+  }
+  return s
+}