feat: breadcrumb trail — faint glowing path showing where you've walked

WASD keys move the camera across the glass platform. As you walk,
a faint accent-colored line and glowing dot markers trace your path,
pulsing gently to match the existing ambient glow aesthetic. Trail
is capped at 80 crumbs; oldest are dropped as you continue moving.

Fixes #142
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.js

@@ -232,6 +232,58 @@ glassPlatformGroup.add(voidLight);
 
 scene.add(glassPlatformGroup);
 
+// === WALKING (WASD) ===
+const walkOffset = new THREE.Vector3(0, 0, 0);
+const walkCamPos = new THREE.Vector3();
+const WALK_SPEED = 0.07;
+const WALK_CLAMP = 4.0;
+const walkKeys = { w: false, a: false, s: false, d: false };
+
+document.addEventListener('keydown', (e) => {
+  if (e.key === 'w' || e.key === 'W') walkKeys.w = true;
+  if (e.key === 'a' || e.key === 'A') walkKeys.a = true;
+  if (e.key === 's' || e.key === 'S') walkKeys.s = true;
+  if (e.key === 'd' || e.key === 'D') walkKeys.d = true;
+});
+document.addEventListener('keyup', (e) => {
+  if (e.key === 'w' || e.key === 'W') walkKeys.w = false;
+  if (e.key === 'a' || e.key === 'A') walkKeys.a = false;
+  if (e.key === 's' || e.key === 'S') walkKeys.s = false;
+  if (e.key === 'd' || e.key === 'D') walkKeys.d = false;
+});
+
+// === BREADCRUMB TRAIL ===
+const CRUMB_MAX = 80;
+const CRUMB_MIN_DIST_SQ = 0.12; // ~0.35 units min gap between crumbs
+/** @type {THREE.Vector3[]} */
+const crumbs = [];
+let crumbLastX = 0;
+let crumbLastZ = 0;
+
+// Faint line tracing the path walked
+const trailGeo = new THREE.BufferGeometry();
+const trailMat = new THREE.LineBasicMaterial({
+  color: NEXUS.colors.accent,
+  transparent: true,
+  opacity: 0.22,
+  depthWrite: false,
+});
+const trailLine = new THREE.Line(trailGeo, trailMat);
+scene.add(trailLine);
+
+// Glowing dots — individual breadcrumb markers
+const crumbDotGeo = new THREE.BufferGeometry();
+const crumbDotMat = new THREE.PointsMaterial({
+  color: NEXUS.colors.accent,
+  size: 0.22,
+  sizeAttenuation: true,
+  transparent: true,
+  opacity: 0.45,
+  depthWrite: false,
+});
+const crumbDots = new THREE.Points(crumbDotGeo, crumbDotMat);
+scene.add(crumbDots);
+
 // === MOUSE-DRIVEN ROTATION ===
 let mouseX = 0;
 let mouseY = 0;
@@ -443,11 +495,39 @@ function animate() {
   requestAnimationFrame(animate);
   const elapsed = clock.getElapsedTime();
 
+  // Update walking (only when in normal ground-level view)
+  if (!overviewMode && !photoMode) {
+    if (walkKeys.w) walkOffset.z -= WALK_SPEED;
+    if (walkKeys.s) walkOffset.z += WALK_SPEED;
+    if (walkKeys.a) walkOffset.x -= WALK_SPEED;
+    if (walkKeys.d) walkOffset.x += WALK_SPEED;
+    walkOffset.x = Math.max(-WALK_CLAMP, Math.min(WALK_CLAMP, walkOffset.x));
+    walkOffset.z = Math.max(-WALK_CLAMP, Math.min(WALK_CLAMP, walkOffset.z));
+  }
+
   // Smooth camera transition for overview mode
   const targetT = overviewMode ? 1 : 0;
   overviewT += (targetT - overviewT) * 0.04;
-  camera.position.lerpVectors(NORMAL_CAM, OVERVIEW_CAM, overviewT);
-  camera.lookAt(0, 0, 0);
+  walkCamPos.set(NORMAL_CAM.x + walkOffset.x, NORMAL_CAM.y, NORMAL_CAM.z + walkOffset.z);
+  camera.position.lerpVectors(walkCamPos, OVERVIEW_CAM, overviewT);
+  camera.lookAt(walkOffset.x * (1 - overviewT), 0, walkOffset.z * (1 - overviewT));
+
+  // Record breadcrumb when player has moved far enough
+  const crumbFx = walkOffset.x;
+  const crumbFz = walkOffset.z;
+  const crumbDx = crumbFx - crumbLastX;
+  const crumbDz = crumbFz - crumbLastZ;
+  if (crumbDx * crumbDx + crumbDz * crumbDz >= CRUMB_MIN_DIST_SQ) {
+    crumbLastX = crumbFx;
+    crumbLastZ = crumbFz;
+    crumbs.push(new THREE.Vector3(crumbFx, 0.015, crumbFz));
+    if (crumbs.length > CRUMB_MAX) crumbs.shift();
+    if (crumbs.length >= 2) trailGeo.setFromPoints(crumbs);
+    crumbDotGeo.setFromPoints(crumbs);
+  }
+  // Pulse the trail glow
+  trailMat.opacity = 0.14 + Math.sin(elapsed * 1.1) * 0.08;
+  crumbDotMat.opacity = 0.35 + Math.sin(elapsed * 1.9) * 0.13;
 
   // Slow auto-rotation — suppressed during overview and photo mode
   const rotationScale = photoMode ? 0 : (1 - overviewT);