perf: reduce GC pressure in render loops

- Pre-compute star RGB components to avoid 3 string.replace() calls per
  star per frame (~40K string allocs/sec eliminated)
- Cache floor gradient object; rebuild only on canvas resize
- Batch CAVA bar DOM updates through requestAnimationFrame so SSE events
  never trigger mid-frame style mutations

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

plugin/contents/ui/dancer.html

@@ -794,16 +794,19 @@ const ctx    = canvas.getContext('2d');
 const STAR_COUNT = 220;
 const stars = [];
 
+const STAR_COLORS = [[255,221,170],[204,216,255],[238,238,255]];
+
 function initStars() {
   stars.length = 0;
   for (let i = 0; i < STAR_COUNT; i++) {
+    const c = STAR_COLORS[Math.random() < 0.15 ? 0 : Math.random() < 0.5 ? 1 : 2];
     stars.push({
       x:     Math.random() * canvas.width,
       y:     Math.random() * canvas.height * 0.62,
       r:     Math.random() * 1.6 + 0.2,
       phase: Math.random() * Math.PI * 2,
       freq:  0.4 + Math.random() * 1.2,
-      color: Math.random() < 0.15 ? '#ffddaa' : (Math.random() < 0.5 ? '#ccd8ff' : '#eeeeff'),
+      cr: c[0], cg: c[1], cb: c[2],
     });
   }
 }
@@ -818,13 +821,10 @@ function drawStars(ts) {
   ctx.clearRect(0, 0, canvas.width, canvas.height);
   const t = ts * 0.001;
   stars.forEach(s => {
-    const alpha = 0.25 + 0.75 * (0.5 + 0.5 * Math.sin(t * s.freq + s.phase));
+    const alpha = (0.25 + 0.75 * (0.5 + 0.5 * Math.sin(t * s.freq + s.phase))).toFixed(2);
     ctx.beginPath();
     ctx.arc(s.x, s.y, s.r, 0, Math.PI * 2);
-    ctx.fillStyle = s.color.replace(')', `, ${alpha.toFixed(2)})`)
+    ctx.fillStyle = `rgba(${s.cr},${s.cg},${s.cb},${alpha})`;
-      .replace('rgb', 'rgba').replace('#ccd8ff', `rgba(204,216,255,${alpha.toFixed(2)})`)
-      .replace('#eeeeff', `rgba(238,238,255,${alpha.toFixed(2)})`)
-      .replace('#ffddaa', `rgba(255,221,170,${alpha.toFixed(2)})`);
     ctx.fill();
   });
   requestAnimationFrame(drawStars);
@@ -845,6 +845,8 @@ const V_LINES     = 24;
 const H_LINES     = 24;
 let   floorScroll = 0;
 let   lastFloorTs = 0;
+let   floorFadeGrad = null;
+let   floorFadeGradH = 0;
 
 function resizeFloor() {
   floorCanvas.width  = window.innerWidth;
@@ -891,11 +893,14 @@ function drawFloor(ts) {
     floorCtx.stroke();
   }
 
-  // Fade near horizon to hide sub-pixel lines
+  // Fade near horizon to hide sub-pixel lines (gradient cached, only rebuilt on resize)
-  const fade = floorCtx.createLinearGradient(0, 0, 0, H * 0.3);
+  if (!floorFadeGrad || floorFadeGradH !== H) {
-  fade.addColorStop(0, '#000814');
+    floorFadeGradH = H;
-  fade.addColorStop(1, 'rgba(0,8,20,0)');
+    floorFadeGrad = floorCtx.createLinearGradient(0, 0, 0, H * 0.3);
-  floorCtx.fillStyle = fade;
+    floorFadeGrad.addColorStop(0, '#000814');
+    floorFadeGrad.addColorStop(1, 'rgba(0,8,20,0)');
+  }
+  floorCtx.fillStyle = floorFadeGrad;
   floorCtx.fillRect(0, 0, W, H * 0.3);
 
   requestAnimationFrame(drawFloor);
@@ -983,22 +988,29 @@ function detectBeat(vals) {
   bassEma    = bassEma    * 0.96 + bass * 0.04;
 }
 
-// Called with semicolon-separated CAVA values (0–100 per bar)
+// Called with semicolon-separated CAVA values (0–100 per bar) — SSE may fire >60fps,
+// so we buffer and flush once per animation frame to avoid mid-frame DOM thrashing.
+let pendingCava = null;
+
 function updateBars(raw) {
-  if (!isPlaying) return;
+  pendingCava = raw;
-  const vals = raw.split(';').map(v => Math.min(100, parseInt(v, 10) || 0));
-  detectBeat(vals);
-
-  // VU meter (low bars)
-  vuBars.forEach((bar, i) => {
-    const v = vals[i] || 0;
-    bar.style.height  = Math.max(3, (v / 100) * MAX_BAR_H) + 'px';
-    bar.style.opacity = 0.3 + 0.7 * (v / 100);
-  });
-
-
 }
 
+function flushBars() {
+  if (pendingCava && isPlaying) {
+    const vals = pendingCava.split(';').map(v => Math.min(100, parseInt(v, 10) || 0));
+    detectBeat(vals);
+    vuBars.forEach((bar, i) => {
+      const v = vals[i] || 0;
+      bar.style.height  = Math.max(3, (v / 100) * MAX_BAR_H) + 'px';
+      bar.style.opacity = 0.3 + 0.7 * (v / 100);
+    });
+    pendingCava = null;
+  }
+  requestAnimationFrame(flushBars);
+}
+requestAnimationFrame(flushBars);
+
 /* ============================================================
    MUSIC-LIGHT THEME (album color)
    ============================================================ */