perf: smoother video player — rAF-driven scrub bar, less cold-start stutter

scrub fill/time were driven off timeupdate, which browsers fire irregularly (a handful of times a second) — visible as the bar stepping instead of gliding. now driven by requestAnimationFrame while playing (display refresh rate), with a direct sync on pause/seeked so it still tracks immediately when not running.\n\npreload=none meant zero data was fetched until the play button was pressed, causing a visible stall right at playback start. preload=metadata lets the browser prime the connection and read the moov atom ahead of time without fetching the full video for visitors who never press play.

assets/js/player.js

@@ -23,14 +23,30 @@
     if (big) big.addEventListener("click", toggle);
     v.addEventListener("click", toggle);
 
-    v.addEventListener("play", function () { root.classList.add("is-playing"); if (play) play.textContent = "❚❚"; });
-    v.addEventListener("pause", function () { root.classList.remove("is-playing"); if (play) play.textContent = "▶"; });
-    v.addEventListener("loadedmetadata", function () { if (time) time.textContent = "00:00 / " + fmt(v.duration); });
-    v.addEventListener("timeupdate", function () {
+    // drive the scrub bar off rAF instead of `timeupdate` — timeupdate only
+    // fires a handful of times a second (browser-dependent), which made the
+    // fill visibly step instead of glide; rAF ties it to the display's own
+    // refresh rate, only running while actually playing.
+    var raf = null;
+    function sync() {
       var p = v.duration ? v.currentTime / v.duration : 0;
       if (fill) fill.style.width = (p * 100) + "%";
       if (time) time.textContent = fmt(v.currentTime) + " / " + fmt(v.duration || 0);
+    }
+    function tick() { sync(); raf = requestAnimationFrame(tick); }
+
+    v.addEventListener("play", function () {
+      root.classList.add("is-playing"); if (play) play.textContent = "❚❚";
+      if (!raf) raf = requestAnimationFrame(tick);
     });
+    v.addEventListener("pause", function () {
+      root.classList.remove("is-playing"); if (play) play.textContent = "▶";
+      if (raf) { cancelAnimationFrame(raf); raf = null; }
+      sync();   // one last sync so the bar lands exactly on the paused frame
+    });
+    v.addEventListener("ended", function () { if (raf) { cancelAnimationFrame(raf); raf = null; } });
+    v.addEventListener("loadedmetadata", function () { if (time) time.textContent = "00:00 / " + fmt(v.duration); });
+    v.addEventListener("seeked", sync);   // scrubbing while paused still needs to reflect immediately
     if (scrub) scrub.addEventListener("click", function (e) {
       var r = scrub.getBoundingClientRect();
       if (v.duration) v.currentTime = ((e.clientX - r.left) / r.width) * v.duration;