Fluid backdrop behind the 3D model

New 'Sfondo Modello 3D' selector (Media tab): gradient or any of the
five fluid modes. The fluid sim paints the canvas first, then the model
renders on top clearing only the depth buffer (overlay mode skips its
own backdrop pass). Palette and speed are shared with the preset;
persisted and re-sent when the output restarts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/model3d.js

@@ -929,7 +929,9 @@ class ModelSim {
   // pose (optional, from the camera-interactive mode): { yaw, leanX, hopY,
   // squash, rim } — extra rotation, sideways lean, jump height (in radii),
   // vertical squash & stretch, touch-glow 0..1.
-  render(timeSec, audio, e, canvas, pose) {
+  // overlay = true: another engine (e.g. the fluid sim) already painted the
+  // canvas this frame — draw only the model on top (clear depth, no backdrop).
+  render(timeSec, audio, e, canvas, pose, overlay) {
     const gl = this.gl;
     const speed = Math.min(2.5, e.speed || 1);
     const mix = e.audioMix !== undefined ? e.audioMix : 1;
@@ -938,10 +940,15 @@ class ModelSim {
 
     gl.viewport(0, 0, canvas.width, canvas.height);
     gl.disable(gl.BLEND);
-    gl.clearColor(0, 0, 0, 1);
-    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
+    if (overlay) {
+      gl.clear(gl.DEPTH_BUFFER_BIT);
+    } else {
+      gl.clearColor(0, 0, 0, 1);
+      gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
+    }
 
     // backdrop
+    if (!overlay) {
     gl.useProgram(this.progBg);
     gl.bindBuffer(gl.ARRAY_BUFFER, this.quad);
     gl.enableVertexAttribArray(this.aBg);
@@ -953,6 +960,7 @@ class ModelSim {
     gl.uniform1f(this.ub.uBass, bass);
     gl.uniform1f(this.ub.uBeat, beat);
     gl.drawArrays(gl.TRIANGLES, 0, 3);
+    }
 
     // model
     const asp = canvas.width/Math.max(1, canvas.height);