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/bloom_down.frag

@@ -0,0 +1,27 @@
+// 13-tap downsample (Jimenez), with a soft threshold on the first level
+in vec2 v_uv;
+out vec4 o_color;
+uniform sampler2D u_src;
+uniform vec2 u_texel;
+uniform float u_threshold;     // <0: no threshold
+void main() {
+  vec2 t = u_texel;
+  vec3 a = texture(u_src, v_uv + t * vec2(-2, 2)).rgb, b = texture(u_src, v_uv + t * vec2(0, 2)).rgb, c = texture(u_src, v_uv + t * vec2(2, 2)).rgb;
+  vec3 d = texture(u_src, v_uv + t * vec2(-2, 0)).rgb, e = texture(u_src, v_uv).rgb, f = texture(u_src, v_uv + t * vec2(2, 0)).rgb;
+  vec3 g = texture(u_src, v_uv + t * vec2(-2, -2)).rgb, h = texture(u_src, v_uv + t * vec2(0, -2)).rgb, i = texture(u_src, v_uv + t * vec2(2, -2)).rgb;
+  vec3 j = texture(u_src, v_uv + t * vec2(-1, 1)).rgb, k = texture(u_src, v_uv + t * vec2(1, 1)).rgb;
+  vec3 l = texture(u_src, v_uv + t * vec2(-1, -1)).rgb, m = texture(u_src, v_uv + t * vec2(1, -1)).rgb;
+  a = min(a, vec3(4096.0)); b = min(b, vec3(4096.0)); c = min(c, vec3(4096.0)); d = min(d, vec3(4096.0)); e = min(e, vec3(4096.0));
+  f = min(f, vec3(4096.0)); g = min(g, vec3(4096.0)); h = min(h, vec3(4096.0)); i = min(i, vec3(4096.0)); j = min(j, vec3(4096.0));
+  k = min(k, vec3(4096.0)); l = min(l, vec3(4096.0)); m = min(m, vec3(4096.0));
+  vec3 col = e * 0.125 + (a + c + g + i) * 0.03125 + (b + d + f + h) * 0.0625 + (j + k + l + m) * 0.125;
+  if (u_threshold >= 0.0) {
+    float br = max(col.r, max(col.g, col.b));
+    float knee = u_threshold * 0.5;
+    float soft = clamp(br - u_threshold + knee, 0.0, 2.0 * knee);
+    soft = soft * soft / (4.0 * knee + 1e-4);
+    float contrib = max(soft, br - u_threshold) / max(br, 1e-4);
+    col *= contrib;
+  }
+  o_color = vec4(sane(col), 1.0);
+}