Optimize object allocation in kids scoring loop

- Replaced array allocation and loop with unrolled scalar comparisons in `kidsScorerStage`.
- Added unit tests and benchmark.
- Measured ~12% performance improvement (282ms -> 249ms for 200k items).

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

tests/feed-engine/kids-scorer.test.mjs

@@ -0,0 +1,73 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { createKidsScorerStage } from "../../js/feedEngine/kidsScoring.js";
+
+test("kids scorer correctly identifies dominant positive component", async () => {
+  const stage = createKidsScorerStage({
+      weights: {
+          w_age: 0.2,
+          w_edu: 0.2,
+          w_author: 0.2,
+          w_pop: 0.2,
+          w_fresh: 0.2,
+          w_risk: 0
+      }
+  });
+
+  // Create an item where one component is clearly dominant
+  // Age appropriateness: 1.0 (perfect duration + tags)
+  // Others: 0
+  const items = [
+    {
+      video: {
+        id: "v1",
+        pubkey: "pk1",
+        duration: 300, // 5 min (perfect for toddler/preschool)
+        tags: [["t", "preschool"], ["t", "learning"]], // Matches preferred
+        created_at: Date.now() / 1000 - 10000000, // Old
+        viewCount: 0
+      },
+      metadata: {},
+    },
+  ];
+
+  const why = [];
+  const context = {
+      runtime: {
+          ageGroup: "preschool",
+          trustedAuthors: []
+      },
+      addWhy: (detail) => why.push(detail)
+  };
+
+  await stage(items, context);
+
+  assert.equal(why.length, 1);
+  assert.equal(why[0].reason, "age-appropriateness");
+  assert.ok(why[0].value > 0);
+});
+
+test("kids scorer picks educational-boost when dominant", async () => {
+  const stage = createKidsScorerStage({
+    educationalTags: ["math"],
+    weights: { w_age: 0, w_edu: 1, w_author: 0, w_pop: 0, w_fresh: 0, w_risk: 0 }
+  });
+
+  const items = [{
+      video: {
+          id: "v2",
+          tags: [["t", "math"]],
+          duration: 300
+      },
+      metadata: {}
+  }];
+
+  const why = [];
+  await stage(items, {
+      runtime: { educationalTags: ["math"] },
+      addWhy: (d) => why.push(d)
+  });
+
+  assert.equal(why.length, 1);
+  assert.equal(why[0].reason, "educational-boost");
+});