clouds3d: closer cruise, sun-alt-driven stars

- Cruise altitude 2.6 -> 2.0 (just above cloud ceiling y=1.5,
  gives the 'just above the clouds' airliner feel)
- Expose __currentSunAlt (radians) from animate() so the
  renderer can tell night from day without misreading
  realDayProgress (which mixes pre-dawn + post-sunset hours)
- Replace (1 - uDayProgress) star gate by smoothstep over uSunAlt:
  nightFactor = 1 when sun is well below horizon, 0 when above
- Widen sky band so stars are visible from the horizon upward
  (cockpit-perspective frame is mostly below rd.y=0.3)

vendor/clouds3d.js

@@ -61,6 +61,7 @@ var FRAG_SRC = [
     'uniform float uSpeed;',
     'uniform float uDensity;',
     'uniform float uDayProgress;',
+    'uniform float uSunAlt;',  // radians, negative = below horizon (night)
     // 3D hash and value noise
     'float hash3(vec3 p) {',
         'p = fract(p * vec3(0.1031, 0.1030, 0.0973));',
@@ -121,9 +122,9 @@ var FRAG_SRC = [
         // pow(...,8) keeps dives rare: most of the time diveT≈0,
         // and only spikes briefly to 1.
         'float diveT = pow(0.5 + 0.5 * sin(uTime * 0.10), 8.0);',
-        // Cruise at y=2.6 (well above cloud ceiling y=1.5), dive to
+        // Cruise at y=2.0 (just above cloud ceiling y=1.5), dive to
         // y=1.2 — close enough to skim the cloud top, never deep inside.
-        'float camY = mix(2.6, 1.2, diveT);',
+        'float camY = mix(2.0, 1.2, diveT);',
         'vec3 ro = vec3(drift, camY, drift * 0.7);',
         // Ray direction: nearly level, like looking out a cockpit
         // window. Horizon sits around the middle of the frame.
@@ -132,7 +133,12 @@ var FRAG_SRC = [
         'float skyT = clamp(rd.y * 0.7 + 0.5, 0.0, 1.0);',
         'vec3 col = mix(uBg, uSky, skyT);',
         // Stars: hash-grid sphere, above horizon, only at night.
-        'float stars = starField(rd) * smoothstep(0.05, 0.35, rd.y) * (1.0 - uDayProgress);',
+        // Night = sun altitude below 0. Visibility extends well into the
+// lower half of the sky so the airplane-cockpit view shows stars
+// from the horizon up.
+        'float nightFactor = smoothstep(0.05, -0.15, uSunAlt);',
+        'float skyMask = smoothstep(-0.10, 0.30, rd.y);',
+        'float stars = starField(rd) * skyMask * nightFactor;',
         'col = mix(col, uLight, clamp(stars, 0.0, 1.0) * 0.9);',
         // Density threshold: weather 'sunny' -> high (sparse clouds),
         // 'stormy' -> low (overcast deck).
@@ -191,12 +197,14 @@ function init(canvas) {
     var uSpeed = gl.getUniformLocation(prog, 'uSpeed');
     var uDensity = gl.getUniformLocation(prog, 'uDensity');
     var uDayProgress = gl.getUniformLocation(prog, 'uDayProgress');
+    var uSunAlt = gl.getUniformLocation(prog, 'uSunAlt');
 
     return {
         gl: gl, prog: prog, buf: buf, aPos: aPos,
         uRes: uRes, uTime: uTime, uSky: uSky, uCloud: uCloud,
         uCloudShadow: uCloudShadow, uLight: uLight, uBg: uBg,
-        uSpeed: uSpeed, uDensity: uDensity, uDayProgress: uDayProgress
+        uSpeed: uSpeed, uDensity: uDensity, uDayProgress: uDayProgress,
+        uSunAlt: uSunAlt
     };
 }
 
@@ -279,6 +287,10 @@ window.clouds3dRender = function(target) {
         ? Math.max(0, Math.min(1, dayProgress)) : 0.5;
     gl.uniform1f(s.uDayProgress, day);
 
+    var sunAlt = (typeof window.__currentSunAlt === 'number')
+        ? window.__currentSunAlt : 0;
+    gl.uniform1f(s.uSunAlt, sunAlt);
+
     var speed = (typeof themeSpeed === 'number')
         ? Math.max(0.1, Math.min(3, themeSpeed)) : 1.0;
     gl.uniform1f(s.uSpeed, speed);