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/README.md

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+# ludic.lab
+
+The visual lab: one model or one mechanic on a **plate**, photographed from fixed cameras. A lab
+scene is a minimal, isolated, reproducible piece of a game - never the whole world. The plate is a
+flat, lit disc 40 m across with a line every metre, a small sky of its own with the sun 40 degrees
+up (one fixed hour), and a clock of frames: 60 a second, whatever the machine. Nothing else is
+loaded - no terrain, no grass, no water (`r3d_plate_mode` in ludic.render3d) - so a scene starts in
+about a second and takes about 120 MB resident (420 MB peak footprint with the GPU driver's share)
+where a lab on a whole valley took 7.8 GB.
+
+`module ludic_lab uses ludic_render3d, ludic_base`.
+
+## A lab program
+
+```ludic
+import "ludic.lab"
+program Lab {
+  numbers float
+  function probe_setup() -> void {
+    lab_model(lab_plate_dir(), "probe.gltf", "probe", 0.0, 0.0, 0.5)
+    lab_shot_at("front", 0.0, 0.7, 0.0, 4.0, 0.0, 12.0)
+    lab_shot_at("above", 0.0, 0.5, 0.0, 7.0, 30.0, 55.0)
+  }
+  def LabScenes probe { setup: fn probe_setup }
+  handler Boot phase Start { lab_boot() }
+  handler Look phase Input { lab_input() }
+  handler Draw phase Render { lab_render() }
+}
+```
+
+`example/plate.ludic` is this program. Scenes are an open registry, so a game - or a package
+showing its own models - adds them from its own module with `def LabScenes <name> { ... }`:
+
+| field | |
+| --- | --- |
+| `setup: fn()` | build what the scene shows and name its cameras |
+| `tick: fn(float)` | move it, by the lab's fixed step (1/60 s) |
+| `draw: fn()` | draw what is not an actor or a scatter layer (actors and layers are drawn already) |
+
+`R3D_SCENE=<name>` picks the scene (the first one added otherwise); an unknown name lists them.
+
+## Running it
+
+- **Headless** (`ludic build ... --headless`): every shot is held `lab_settle` frames (45), written
+  to `build/lab/<scene>/<shot>.png`, and after the last the program quits. The headless loop reads a
+  line of stdin a frame: `yes '' | head -n 20000 | R3D_SCENE=probe ./lab`.
+- **In a window**: N and P step through the shots.
+- `tools/lab/run.sh <lab.ludic> <scene> ...` (in the toolchain's checkout) builds the program, runs
+  each scene and makes `build/lab/<scene>/sheet.png`, a contact sheet (`tools/lab/sheet.py`).
+
+## API
+
+| | |
+| --- | --- |
+| `lab_boot()`, `lab_input()`, `lab_render()` | the program's Start, Input and Render handlers |
+| `lab_shot(name, x, y, z, yaw_deg, pitch_deg)` | a camera at a point, looking along yaw and pitch (degrees) |
+| `lab_shot_at(name, tx, ty, tz, dist, from_deg, up_deg)` | a camera `dist` metres from a point, from a bearing (0 = from +z), `up_deg` above it, looking at it |
+| `lab_shot_count()` | the cameras named so far |
+| `lab_model(dir, file, node, x, z, yaw) -> Actor`, `lab_stand(a, x, z, yaw)`, `lab_ground_y(x, z)` | a glTF node stood on the plate; the plate is flat at y = 0 |
+| `lab_plate_dir()` | the package's `plate/`: the disc, its sky and the probe model |
+| `lab_settle(frames)`, `lab_font(dir)`, `lab_title(s)`, `lab_note(s)` | how long a shot settles; the overlay font for captions (none by default); the window's title; a line under the caption |
+| `lab_scene()`, `lab_frame()`, `lab_time()` | the scene, and the lab's clock |
+| `lab_ppm_to_png(ppm, png)` | a frame as a PNG (stored, not compressed) |
+
+## The plate's files
+
+`plate/` is built by `tools/lab/plate_build.py` in the toolchain's checkout and is deterministic:
+`plate.gltf` (the disc), `probe.gltf` (a box with a ball on it, for looking at the light), and
+`sky.hdr` (512 x 256, the sun at 40 degrees). Change the script and run it again; never edit them.
+
+## Not here yet
+
+- A heightfield patch for a scene that needs a slope: the plate is flat.