Stage 3.6 — models visible: transparent model compositing + Assembly density

Model layers render with transparent clear so Compositor lumakey/normal
can show composition underneath where no mesh covers — Assembly is now
sparse (hero + satellites on ground, not opaque fill), satellites wired
to count/spread/density so the density slider visibly changes the stage
and a seek is still just f(t,seed). Shared camera plumbing already
drove framing->dolly/personality->drift for real parallax.

Gates: lint 108 clean / 70 literals / 69 scenes; vite built.

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

flow-state/src/engine/Layer.js

@@ -336,6 +336,14 @@ export class ModelLayer extends Layer {
     constructor(options) {
         super(options);
         this.scene = new THREE.Scene();
+        // Model layers are sparse: a hero plus a few satellites on a ground.
+        // Mark the scene for a transparent clear so Renderer clears to alpha 0
+        // and Compositor's lumakey/normal blend shows the composition underneath
+        // where there is no mesh — same role as a 'composable' shader layer.
+        this.scene.userData.transparentBackground = true;
+        // Keep the WebGL clear colour transparent as well; renderScene reads the
+        // scene flag but the fallback alpha is set here for safety.
+        this.scene.background = null;
         this.camera = new THREE.PerspectiveCamera(60, 16 / 9, 0.1, 200);
         this.camera.position.set(0, 0, 5);
         this.actorSpec = options.actorSpec || null;