feat(gl): OpenGL 4.1 and the ludic.render3d renderer

`Gl.*` binds the whole OpenGL 4.1 core API — every entry point of the
platform gl3.h with every GL_* constant, generated by `ludic-dev glgen`
with per-call ABI thunks. Windowed builds get an NSOpenGLContext on the
existing window at Retina resolution; headless builds render into an
offscreen CGL context, so a program that uses Gl.* renders and
screenshots identically under the test harness. It links gl.ll, the
thunks and OpenGL.framework only when used; every other build stays
byte-identical.

packages/ludic.render3d is a physically based renderer written on that
surface: HDRI image-based lighting, GPU-generated terrain with scanned
PBR materials, CDLOD, cascaded shadows, glTF with skinning, instanced
vegetation with impostors, procedural grass, water, SSAO, and an HDR
pipeline with bloom, auto-exposure and ACES.

It also carries this session's work on it: the terrain at half its cost
(10.3 -> 5.4 ms of frame), the streaming hitch that got worse the longer
you played, a resize that emptied the world, and the packaging that lets
a game use the renderer from its own repository — `ludic assets`, the
material manifest shipping with the package, and shader lookup falling
back to the install root. See changes/ for each, with its numbers.

The camping game that drove all of it has moved out to its own
repository, Maroon Lake; examples/rendering/smooth.ludic stays as the
renderer's example here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/ludic-cli/assets.ludic

@@ -0,0 +1,73 @@
+# assets.ludic — fetch the CC0 Poly Haven assets the renderer uses
+# (ported off tools/glgen/fetch_assets.sh, the last shell script in the tree).
+#
+#   ludic assets [--force]         (a game, fetching what the renderer needs)
+#   ludic dev fetch-assets [--force]   (the same thing, from a Ludic checkout)
+#
+# The manifest is `packages/ludic.render3d/assets.manifest` — one `<relative path> <url>`
+# pair per line — and it belongs to the renderer, not to any one project: the renderer
+# decides which scanned materials it wants. It ships with the package (and so with the
+# toolchain), so a game outside this repo fetches the right set without keeping its own
+# copy of the list. The files land in the PROJECT, under assets/polyhaven/: they are
+# large and redistributable from their origin, so they are fetched rather than tracked
+# by anyone.
+
+# the manifest that belongs to the renderer, in a checkout or in the install
+function assets_manifest_path() -> pointer {
+  let local = "packages/ludic.render3d/assets.manifest"
+  if file_exists(local) { return local }
+  var home = getenv("LUDIC_HOME")
+  if home == null or slen(home) == 0 { home = `{getenv_or("HOME", "")}/.ludic` }
+  let inst = `{home}/packages/ludic.render3d/assets.manifest`
+  if file_exists(inst) { return inst }
+  return ""
+}
+
+function cmd_fetch_assets() -> int {
+  let force = arg_count() > 2 and arg(2) == "--force"
+  let root = "assets/polyhaven"
+  let mpath = assets_manifest_path()
+  if mpath == "" { err("ludic assets: cannot find packages/ludic.render3d/assets.manifest (is the toolchain installed?)\n");  return 1 }
+  let manifest = read_file(mpath)
+  if manifest == null { err(`ludic assets: cannot read {mpath}\n`);  return 1 }
+
+  var got = 0
+  var had = 0
+  var failed = 0
+  let n = slen(manifest)
+  var i = 0
+  while i < n {
+    var j = i
+    while j < n and manifest[j] != 10 { j += 1 }
+    let line = Text.trim(str_sub(manifest, i, j))
+    i = j + 1
+    if slen(line) == 0 { continue }
+
+    # split the line into <relative path> <url> on the first run of whitespace
+    let ln = slen(line)
+    var k = 0
+    while k < ln and not str_space(line[k]) { k += 1 }
+    let rel = str_sub(line, 0, k)
+    while k < ln and str_space(line[k]) { k += 1 }
+    let url = Text.trim(str_sub(line, k, ln))
+    if slen(rel) == 0 or slen(url) == 0 { continue }
+
+    let dest = `{root}/{rel}`
+    if not force and shq(`test -s {dest}`) {
+      had += 1
+      continue
+    }
+    print(`fetch {rel}`)
+    run(`mkdir -p "$(dirname {dest})"`)
+    if not shq(`curl -sSL --retry 3 -o {dest} {url}`) {
+      err(`fetch-assets: cannot fetch {rel}\n`)
+      run(`rm -f {dest}`)
+      failed += 1
+    } else {
+      got += 1
+    }
+  }
+  if failed > 0 { err(`fetch-assets: {string(failed)} failed\n`);  return 1 }
+  print(`OK {string(got)} fetched, {string(had)} already present`)
+  return 0
+}