feat: add breadcrumb trail showing glowing path through star field

Tracks camera position in star-local space at 0.25s intervals, rendering
faint cyan-blue glowing dots that fade over 12 seconds. Crumbs rotate with
the star field so they stay embedded in space as you explore.

Fixes #142

app.js

@@ -126,6 +126,71 @@ window.addEventListener('resize', () => {
   renderer.setSize(window.innerWidth, window.innerHeight);
 });
 
+// === BREADCRUMB TRAIL ===
+const TRAIL_MAX = 100;
+const TRAIL_LIFETIME = 12; // seconds before a crumb fades out
+const TRAIL_INTERVAL = 0.25; // seconds between crumb placements
+let lastCrumbTime = -TRAIL_INTERVAL;
+
+const trailPosArr = new Float32Array(TRAIL_MAX * 3);
+const trailColArr = new Float32Array(TRAIL_MAX * 3);
+const trailGeo = new THREE.BufferGeometry();
+trailGeo.setAttribute('position', new THREE.BufferAttribute(trailPosArr, 3));
+trailGeo.setAttribute('color', new THREE.BufferAttribute(trailColArr, 3));
+trailGeo.setDrawRange(0, 0);
+
+const trailMat = new THREE.PointsMaterial({
+  size: 2.5,
+  sizeAttenuation: true,
+  transparent: true,
+  opacity: 0.9,
+  vertexColors: true,
+});
+const trailMesh = new THREE.Points(trailGeo, trailMat);
+scene.add(trailMesh);
+
+// crumbs store positions in star-local space so they stay embedded in the star field
+const crumbs = []; // { lx, ly, lz, born }
+
+function addCrumb(elapsed) {
+  // Project camera position into star-local space to track the viewing path
+  const invQ = stars.quaternion.clone().invert();
+  const localCam = camera.position.clone().applyQuaternion(invQ);
+  crumbs.push({ lx: localCam.x, ly: localCam.y, lz: localCam.z, born: elapsed });
+  if (crumbs.length > TRAIL_MAX) crumbs.shift();
+  lastCrumbTime = elapsed;
+}
+
+function updateTrail(elapsed) {
+  // Remove expired crumbs
+  while (crumbs.length > 0 && elapsed - crumbs[0].born > TRAIL_LIFETIME) {
+    crumbs.shift();
+  }
+
+  const n = crumbs.length;
+  for (let i = 0; i < n; i++) {
+    const c = crumbs[i];
+    const age = elapsed - c.born;
+    const t = 1 - age / TRAIL_LIFETIME;
+    const fade = t * t; // quadratic fade for a softer tail
+
+    trailPosArr[i * 3]     = c.lx;
+    trailPosArr[i * 3 + 1] = c.ly;
+    trailPosArr[i * 3 + 2] = c.lz;
+    // Faint cyan-blue glow matching the accent palette
+    trailColArr[i * 3]     = 0.1 * fade;
+    trailColArr[i * 3 + 1] = 0.5 * fade;
+    trailColArr[i * 3 + 2] = 1.0 * fade;
+  }
+  trailGeo.attributes.position.needsUpdate = true;
+  trailGeo.attributes.color.needsUpdate = true;
+  trailGeo.setDrawRange(0, n);
+
+  // Trail rotates with the star field so crumbs stay embedded in space
+  trailMesh.rotation.x = stars.rotation.x;
+  trailMesh.rotation.y = stars.rotation.y;
+}
+
 // === ANIMATION LOOP ===
 const clock = new THREE.Clock();
 
@@ -146,6 +211,12 @@ function animate() {
   // Subtle pulse on constellation opacity
   constellationLines.material.opacity = 0.12 + Math.sin(elapsed * 0.5) * 0.06;
 
+  // Breadcrumb trail — drop a crumb every interval
+  if (elapsed - lastCrumbTime >= TRAIL_INTERVAL) {
+    addCrumb(elapsed);
+  }
+  updateTrail(elapsed);
+
   renderer.render(scene, camera);
 }