Optimize createExploreDiversitySorter from O(N^3) to O(N^2)

This change significantly improves the performance of the `explore-diversity-sorter` by caching and incrementally updating the maximum similarity penalty for each candidate.

Previously, the algorithm re-calculated the cosine similarity between every remaining candidate and *every* selected item in each iteration, resulting in O(N^3) complexity.

The optimized implementation maintains a `candidateState` map. When a new item is selected, we only update the cached penalty for remaining candidates by comparing them against the *newly selected* item, reducing the complexity to O(N^2).

Benchmark results (reproduce_issue.js):
- 1000 items: ~30s -> ~130ms (>200x improvement)
- 500 items: ~3.5s -> ~67ms
- 100 items: ~345ms -> ~33ms

All existing tests pass.

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

reproduce_issue.js

@@ -0,0 +1,44 @@
+
+import { createExploreDiversitySorter } from "./js/feedEngine/sorters.js";
+
+const buildItem = ({ id, score, tags }) => ({
+  video: {
+    id: String(id),
+    tags: tags.map((tag) => ["t", tag]),
+    rootCreatedAt: 1000,
+  },
+  metadata: {
+    exploreScore: score,
+  },
+});
+
+function generateData(count) {
+  const items = [];
+  const tagsPool = ["cats", "dogs", "birds", "nature", "funny", "news", "crypto", "bitcoin", "tech", "music"];
+  for (let i = 0; i < count; i++) {
+    const score = Math.random();
+    // Assign 1-3 random tags
+    const numTags = Math.floor(Math.random() * 3) + 1;
+    const itemTags = [];
+    for (let t = 0; t < numTags; t++) {
+      itemTags.push(tagsPool[Math.floor(Math.random() * tagsPool.length)]);
+    }
+    items.push(buildItem({ id: i, score, tags: itemTags }));
+  }
+  return items;
+}
+
+async function runBenchmark() {
+  const sorter = createExploreDiversitySorter({ lambda: 0.7 });
+  const counts = [100, 500, 1000];
+
+  for (const count of counts) {
+    const items = generateData(count);
+    const start = performance.now();
+    sorter(items, { addWhy() {} });
+    const end = performance.now();
+    console.log(`Count: ${count}, Time: ${(end - start).toFixed(2)}ms`);
+  }
+}
+
+runBenchmark();