Fix boot safely, add social OG image, restore performance gains

Guard fitCanvas until Engine is ready, show a boot placeholder during shader compile, generate a 1200x630 OG image for X and WhatsApp, and point repo meta to lumenshaders.vercel.app.

Co-authored-by: Cursor <cursoragent@cursor.com>

js/engine.js

@@ -5,6 +5,8 @@ var Engine = (function () {
   var canvas, gl, program, uniforms = {};
   var playing = true;
   var suspended = false;
+  var started = false;
+  var ready = false;
   var loopT = 0;            // seconds into current loop
   var lastTick = 0;
   var fps = 60, fpsAcc = 0, fpsN = 0, fpsCb = null;
@@ -32,13 +34,14 @@ var Engine = (function () {
     return sh;
   }
 
-  function init(canvasEl, paramsGetter) {
+  function init(canvasEl, paramsGetter, opts) {
+    opts = opts || {};
     canvas = canvasEl;
     getParams = paramsGetter;
     gl = canvas.getContext("webgl2", {
       antialias: false,
       preserveDrawingBuffer: true,
-      powerPreference: "high-performance"
+      powerPreference: "default"
     });
     if (!gl) throw new Error("WebGL2 not available");
 
@@ -61,14 +64,25 @@ var Engine = (function () {
       uniforms[n] = gl.getUniformLocation(program, n);
     });
 
+    ready = true;
+    lastTick = performance.now();
+    started = false;
+    if (opts.autostart !== false) start();
+  }
+
+  function start() {
+    if (!ready || started) return;
+    started = true;
+    suspended = false;
     lastTick = performance.now();
     requestAnimationFrame(tick);
   }
 
   function setSize(w, h) {
+    if (!canvas) return;
     canvas.width = w;
     canvas.height = h;
-    gl.viewport(0, 0, w, h);
+    if (gl) gl.viewport(0, 0, w, h);
   }
 
   function pushUniforms(P, phase) {
@@ -168,6 +182,8 @@ var Engine = (function () {
 
   return {
     init: init,
+    start: start,
+    isReady: function () { return ready; },
     setSize: setSize,
     renderAt: renderAt,
     readPixels: readPixels,