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/png.ludic

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+# png.ludic - the renderer writes a frame as a PPM; the lab keeps it as a PNG, stored rather than
+# compressed (a PNG anything opens, with no zlib here): the shot is the PPM's pixels, filter 0
+# on every row, in deflate's stored blocks, with its CRCs and its Adler-32.
+var lp_table: []long = null
+
+function lp_u32(hi: int, lo: int) -> long { return long(hi) * long(65536) + long(lo) }
+function lp_table_init() -> void {
+  if lp_table != null { return }
+  lp_table = new []long
+  let poly = lp_u32(60856, 33568)          # 0xEDB88320
+  for n in 0 .. 256 {
+    var c = long(n)
+    for k in 0 .. 8 {
+      if (c & long(1)) != long(0) { c = (c >> long(1)) ^ poly } else { c = c >> long(1) }
+    }
+    push(lp_table, c)
+  }
+}
+# CRC-32 of out[from .. from + n)
+function lp_crc(out: []byte, from: int, n: int) -> long {
+  lp_table_init()
+  let mask = lp_u32(65535, 65535)
+  var c = mask
+  for i in from .. from + n {
+    let k = int((c ^ long(out[i])) & long(255))
+    c = lp_table[k] ^ (c >> long(8))
+  }
+  return c ^ mask
+}
+function lp_put32(out: []byte, at: int, v: long) -> void {
+  out[at] = int((v >> long(24)) & long(255))
+  out[at + 1] = int((v >> long(16)) & long(255))
+  out[at + 2] = int((v >> long(8)) & long(255))
+  out[at + 3] = int(v & long(255))
+}
+function lp_tag(out: []byte, at: int, tag: string) -> void {
+  for i in 0 .. 4 { out[at + i] = tag[i] }
+}
+# a PPM's header: "P6", width, height, maxval, each after white space; returns the pixel offset
+var lp_w: int = 0
+var lp_h: int = 0
+function lp_ppm_header(d: []byte) -> int {
+  var i = 2
+  var got = 0
+  var vals = new []int
+  while got < 3 and i < len(d) {
+    while i < len(d) and (d[i] == 32 or d[i] == 10 or d[i] == 13 or d[i] == 9) { i += 1 }
+    var v = 0
+    while i < len(d) and d[i] >= 48 and d[i] <= 57 {
+      v = v * 10 + (d[i] - 48)
+      i += 1
+    }
+    push(vals, v)
+    got += 1
+  }
+  lp_w = vals[0]
+  lp_h = vals[1]
+  return i + 1
+}
+# the PPM at `ppm` as a PNG at `png`; the PPM is removed
+export function lab_ppm_to_png(ppm: string, png: string) -> bool {
+  let d = Fs.read_bytes(ppm)
+  if d == null or len(d) < 8 { return false }
+  let px = lp_ppm_header(d)
+  let w = lp_w
+  let h = lp_h
+  let row = w * 3 + 1
+  let raw = row * h
+  let blocks = (raw + 65534) / 65535
+  let idat = 2 + blocks * 5 + raw + 4
+  let total = 8 + 25 + 12 + idat + 12
+  let out = buffer(total)
+  let sig = [137, 80, 78, 71, 13, 10, 26, 10]
+  for i in 0 .. 8 { out[i] = sig[i] }
+  var at = 8
+  lp_put32(out, at, long(13))
+  lp_tag(out, at + 4, "IHDR")
+  lp_put32(out, at + 8, long(w))
+  lp_put32(out, at + 12, long(h))
+  out[at + 16] = 8
+  out[at + 17] = 2
+  out[at + 18] = 0
+  out[at + 19] = 0
+  out[at + 20] = 0
+  lp_put32(out, at + 21, lp_crc(out, at + 4, 17))
+  at += 25
+  lp_put32(out, at, long(idat))
+  lp_tag(out, at + 4, "IDAT")
+  var o = at + 8
+  out[o] = 120
+  out[o + 1] = 1
+  o += 2
+  var a = 1
+  var b = 0
+  var left = raw
+  var k = 0                                # the byte of the raw stream being written
+  while left > 0 {
+    let n = Math.min(left, 65535)
+    left -= n
+    var fin = 0
+    if left == 0 { fin = 1 }
+    out[o] = fin
+    out[o + 1] = n & 255
+    out[o + 2] = (n >> 8) & 255
+    out[o + 3] = (~n) & 255
+    out[o + 4] = ((~n) >> 8) & 255
+    o += 5
+    for j in 0 .. n {
+      let y = k / row
+      let x = k % row
+      var v = 0
+      if x > 0 { v = d[px + y * w * 3 + x - 1] }
+      out[o] = v
+      a = (a + v) % 65521
+      b = (b + a) % 65521
+      o += 1
+      k += 1
+    }
+  }
+  lp_put32(out, o, long(b) * long(65536) + long(a))
+  o += 4
+  lp_put32(out, o, lp_crc(out, at + 4, o - at - 4))
+  o += 4
+  lp_put32(out, o, long(0))
+  lp_tag(out, o + 4, "IEND")
+  lp_put32(out, o + 8, lp_crc(out, o + 4, 4))
+  o += 12
+  if not Fs.write_bytes(png, out, o) { return false }
+  Fs.remove(ppm)
+  return true
+}