clouds2d: texture-based 4-octave fBm (fizzer-inspired)

Rewrite clouds2d.js to sample a noise texture instead of computing
fBm with hash functions. Each octave is one texture lookup with
doubled UV scale and halved amplitude.

- Same visual range as before but ~4x cheaper on ALU
- REPEAT-wrapped texture tiles seamlessly when scrolled by time
- Per-pixel hash21 jitter keeps the regular tiling from flickering
- Exposes noise as a uniform, so multiple fragment shaders could
  share it
- Theme-agnostic: pulls colours from __currentPalette

Also adds #renderer=clouds2d URL-hash override for quick A/B
testing between the volumetric clouds3d and the 2D version.

background.html

@@ -479,8 +479,11 @@
     <script>
 window.__clouds2dStarted = true;
 try {
+window.__noiseDataUri = 'data:image/png;base64,PLACEHOLDER';
 var noiseDataUri = 'data:image/png;base64,PLACEHOLDER';
-var renderer = (window.CONFIG && window.CONFIG.renderer) || 'vanta';
+// URL override for easy A/B testing: #renderer=clouds2d / #renderer=clouds3d
+var _urlRenderer = (location.hash.match(/renderer=(\w+)/) || [])[1];
+var renderer = _urlRenderer || (window.CONFIG && window.CONFIG.renderer) || 'vanta';
 var useVanta = (renderer !== 'clouds2d' && renderer !== 'clouds3d');
 var effect;
 if (useVanta && typeof VANTA !== 'undefined') {