Add satellite ground texture (ESRI World Imagery, Passau)

Pulled a 5x4 tile mosaic of the Passau region (zoom 10) from
ESRI's free ArcGIS World Imagery service, stitched + resized to
1024x768 JPEG (277 KB). Added to Dockerfile as ground.jpg.

Shader side:
- New uniform uGround (texture unit 1)
- horizonMask: rd.y in [-0.20, -0.02] -> ground fades in from 0
  to 1 as we look further down
- gUv derived from rd.xz so looking forward/down scrolls across
  the satellite image (parallax-like feel)
- Distant ground fades toward uBg via a 'fog' factor so the horizon
  blends naturally with the sky gradient

Loading is async — clouds render before ground texture arrives.
Falls back to a black sample if ground.jpg fails to load.

vendor/clouds3d.js

@@ -53,6 +53,7 @@ var FRAG_SRC = [
     'precision highp float;',
     'uniform vec2 uResolution;',
     'uniform float uTime;',
+    'uniform sampler2D uGround;',
     'uniform vec3 uSky;',
     'uniform vec3 uCloud;',
     'uniform vec3 uCloudShadow;',
@@ -194,6 +195,18 @@ var FRAG_SRC = [
         // remain visible against a deep but not void-black sky.
         'float skyDarkness = nightFactor * 0.55;',
         'col = mix(col, col * (1.0 - skyDarkness), 1.0 - cloud);',
+        '// Horizon: when looking down (rd.y < 0), sample the satellite
+        // ground texture based on ray direction. Use the horizontal
+        // component as a UV — the further off-axis, the more the
+        // texture scrolls. Adds parallax-like feel even though the
+        // ground is a flat image.
+        'float gmask = smoothstep(-0.02, -0.20, rd.y);',
+        'vec2 gUv = vec2(rd.x * 0.5 + 0.5, -rd.y * 0.5 + 0.3);',
+        'vec3 groundCol = texture2D(uGround, gUv).rgb;',
+        // Distance fog: distant ground fades toward sky colour.
+        'float fog = clamp(-rd.y * 1.5, 0.0, 1.0);',
+        'groundCol = mix(groundCol, uBg, fog * 0.6);',
+        'col = mix(col, groundCol, gmask);',
         'gl_FragColor = vec4(col, 1.0);',
     '}'
 ].join('\n');
@@ -216,6 +229,7 @@ function init(canvas) {
     var aPos = gl.getAttribLocation(prog, 'aPos');
     var uRes = gl.getUniformLocation(prog, 'uResolution');
     var uTime = gl.getUniformLocation(prog, 'uTime');
+    var uGround = gl.getUniformLocation(prog, 'uGround');
     var uSky = gl.getUniformLocation(prog, 'uSky');
     var uCloud = gl.getUniformLocation(prog, 'uCloud');
     var uCloudShadow = gl.getUniformLocation(prog, 'uCloudShadow');
@@ -228,13 +242,37 @@ function init(canvas) {
 
     return {
         gl: gl, prog: prog, buf: buf, aPos: aPos,
-        uRes: uRes, uTime: uTime, uSky: uSky, uCloud: uCloud,
+        uRes: uRes, uTime: uTime, uGround: uGround, uSky: uSky, uCloud: uCloud,
         uCloudShadow: uCloudShadow, uLight: uLight, uBg: uBg,
         uSpeed: uSpeed, uDensity: uDensity, uDayProgress: uDayProgress,
         uSunAlt: uSunAlt
     };
 }
 
+// Load an image and upload it as a CLAMP_TO_EDGE wrapped texture
+// (ground shouldn't tile — it's a real photo).
+function loadImageTexture(gl, src) {
+    return new Promise(function(resolve, reject) {
+        var img = new Image();
+        img.crossOrigin = 'anonymous';
+        img.onload = function() {
+            var tex = gl.createTexture();
+            gl.bindTexture(gl.TEXTURE_2D, tex);
+            gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, true);
+            gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, img);
+            gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
+            gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
+            gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
+            gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
+            resolve(tex);
+        };
+        img.onerror = function() {
+            reject(new Error('clouds3d: failed to load image: ' + src));
+        };
+        img.src = src;
+    });
+}
+
 function hex(rgb) {
     return [((rgb >> 16) & 255) / 255,
             ((rgb >> 8) & 255) / 255,
@@ -272,6 +310,17 @@ window.clouds3dRender = function(target) {
     if (!canvas.__state) {
         canvas.__state = init(canvas);
         if (!canvas.__state) return;
+        // Load the satellite ground texture asynchronously. The
+        // ground isn't strictly needed for the clouds-only view, so
+        // frames render normally before it loads — the shader just
+        // samples a black texture in the meantime.
+        canvas.__groundReady = false;
+        loadImageTexture(s.gl, 'ground.jpg').then(function(tex) {
+            canvas.__ground = tex;
+            canvas.__groundReady = true;
+        }).catch(function(err) {
+            console.warn(err.message);
+        });
     }
 
     var s = canvas.__state;
@@ -285,6 +334,11 @@ window.clouds3dRender = function(target) {
     gl.enableVertexAttribArray(s.aPos);
     gl.vertexAttribPointer(s.aPos, 2, gl.FLOAT, false, 0, 0);
 
+    // Bind ground texture to texture unit 1 (noise is on unit 0).
+    gl.activeTexture(gl.TEXTURE1);
+    gl.bindTexture(gl.TEXTURE_2D, canvas.__ground || null);
+    gl.uniform1i(s.uGround, 1);
+
     gl.uniform2f(s.uRes, canvas.width, canvas.height);
     gl.uniform1f(s.uTime, performance.now() / 1000);