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/boot.ludic

@@ -0,0 +1,56 @@
+# boot.ludic - the window, the renderer on a plate (no terrain, no grass, no water), the plate and
+# its sky, then the scene named by R3D_SCENE (else the first one added)
+function lab_draw() -> void {
+  actor_draw()
+  scatter_draw()
+  if lb_draw != null { lb_draw() }
+}
+function lab_casters(light_vp: floats) -> void {
+  actor_draw_casters(light_vp)
+  scatter_draw_casters(light_vp)
+}
+function lab_pick_scene() -> int {
+  if LAB_COUNT == 0 { return -1 }
+  lb_scene = LabScenes[0].key
+  if Os.has_env("R3D_SCENE") { lb_scene = Os.env("R3D_SCENE") }
+  return labscenes_find(lb_scene)
+}
+# once, from the program's Start handler
+export function lab_boot() -> void {
+  lb_windowed = is_windowed()
+  let k = lab_pick_scene()
+  if k < 0 {
+    print(`lab: no scene called '{lb_scene}' - {lab_scene_names()}`)
+    quit()
+    return
+  }
+  var ww = 1920
+  var wh = 1080
+  if lb_windowed {
+    ww = 960
+    wh = 540
+  }
+  r3d_plate_mode(true)
+  r3d_sky_path = `{lab_plate_dir()}/sky.hdr`
+  if not r3d_init(ww, wh, lb_title) {
+    quit()
+    return
+  }
+  if lb_font != "" { overlay_init(lb_font) }
+  r3d_on_draw(fn lab_draw)
+  r3d_on_casters(fn lab_casters)
+  lab_plate_load()
+  lab_shots_reset()
+  lb_out = `build/lab/{lb_scene}`
+  let sc = LabScenes[k]
+  lb_draw = sc.draw
+  if sc.setup != null { sc.setup() }
+  if lab_shot_count() == 0 { lab_shot_at("plate", 0.0, 1.0, 0.0, 8.0, 0.0, 15.0) }
+  lab_shot_apply(0)
+  if not lb_windowed {
+    Fs.mkdir("build")
+    Fs.mkdir("build/lab")
+    Fs.mkdir(lb_out)
+  }
+  print(`lab: scene {lb_scene}, {lab_shot_count()} shots`)
+}