Refactor ScrollSpy to use IntersectionObserver for better performance

Replaced the scroll event listener and `getBoundingClientRect` polling loop with `IntersectionObserver` to eliminate layout thrashing and reduce main thread work during scrolling.
Verified logic with unit tests and confirmed no regressions in frontend behavior.

Co-authored-by: PR0M3TH3AN <[email protected]>

benchmarks/scroll_spy_bench.js

@@ -0,0 +1,61 @@
+
+import { performance } from 'perf_hooks';
+
+// Mock data
+const NUM_HEADINGS = 100;
+const SCROLL_EVENTS = 10000;
+
+const headings = [];
+for (let i = 0; i < NUM_HEADINGS; i++) {
+  headings.push({
+    id: `heading-${i}`,
+    // Mock getBoundingClientRect
+    getBoundingClientRect: () => ({
+      top: (i * 100) - currentScrollY, // Simple layout simulation
+    })
+  });
+}
+
+let currentScrollY = 0;
+let activeId = "";
+
+// The function to benchmark (copied/adapted from js/docsView.js)
+const updateActiveFromScroll = () => {
+    const offset = 96;
+    let nextId = headings[0]?.id || "";
+
+    // In the real code, this runs on every scroll (throttled by rAF)
+    // We simulate the logic here.
+    for (const heading of headings) {
+      const top = heading.getBoundingClientRect().top - offset;
+      if (top <= 0) {
+        nextId = heading.id;
+      } else {
+        break;
+      }
+    }
+
+    if (nextId && nextId !== activeId) {
+      activeId = nextId;
+      // setActiveSection logic (simplified)
+    }
+};
+
+console.log(`Benchmarking legacy scroll spy logic...`);
+console.log(`Headings: ${NUM_HEADINGS}`);
+console.log(`Scroll events simulated: ${SCROLL_EVENTS}`);
+
+const start = performance.now();
+
+for (let i = 0; i < SCROLL_EVENTS; i++) {
+  // Simulate scrolling down
+  currentScrollY += 5;
+  updateActiveFromScroll();
+}
+
+const end = performance.now();
+const duration = end - start;
+
+console.log(`Total execution time: ${duration.toFixed(2)}ms`);
+console.log(`Average time per scroll event: ${(duration / SCROLL_EVENTS).toFixed(4)}ms`);
+console.log(`Note: This benchmark only measures JS execution time. Real browser performance is worse due to layout thrashing (getBoundingClientRect).`);