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>

packages/ludic.render3d/shaders/impostor.vert

@@ -0,0 +1,42 @@
+// camera-facing (around y) card per instance, showing the atlas tile nearest the view angle
+layout(location = 0) in vec2 a_xy;       // [-0.5, 0.5] x [0, 1]
+layout(location = 1) in vec2 a_uv;
+layout(location = 3) in vec4 i_pos;      // x y z scale
+layout(location = 4) in vec4 i_rot;      // sin cos seed wind
+uniform mat4 u_view;
+uniform mat4 u_proj;
+uniform mat4 u_light_vp;
+uniform vec3 u_cam_pos;
+uniform vec3 u_face_dir;                 // direction the cards face (to the camera, or the sun in the shadow pass)
+uniform float u_radius;
+uniform float u_height;
+uniform float u_tiles;
+out vec2 v_uv;
+out vec3 v_wpos;
+out float v_seed;
+out float v_tile;
+out float v_yaw;
+void main() {
+  vec3 center = i_pos.xyz;
+#ifdef SHADOW_PASS
+  vec3 toCam = normalize(vec3(u_face_dir.x, 0.0, u_face_dir.z));
+#else
+  vec3 toCam = u_cam_pos - center; toCam.y = 0.0; toCam = normalize(toCam);
+#endif
+  vec3 right = vec3(-toCam.z, 0.0, toCam.x);
+  float yaw = atan(i_rot.x, i_rot.y);
+  // angle of the viewer around the (rotated) tree, in tile units
+  float ang = atan(toCam.x, -toCam.z) - yaw;
+  float t = floor(fract(ang / 6.2831853) * u_tiles + 0.5);
+  v_tile = mod(t, u_tiles);
+  v_yaw = yaw;
+  vec3 w = center + right * (a_xy.x * 2.0 * u_radius * i_pos.w) + vec3(0.0, a_xy.y * u_height * i_pos.w, 0.0);
+  v_wpos = w;
+  v_uv = a_uv;
+  v_seed = i_rot.z;
+#ifdef SHADOW_PASS
+  gl_Position = u_light_vp * vec4(w, 1.0);
+#else
+  gl_Position = u_proj * u_view * vec4(w, 1.0);
+#endif
+}