Fix test failures: HMAC length check (P3), fast-check fround (P5)

- P3: timingSafeEqual requires equal-length buffers; add length guard before compare
- P5: fast-check fc.float requires 32-bit floats; wrap min with Math.fround()
- All 5 Node products: 83 tests passing across 13 test files

products/05-aws-cost-anomaly/tests/unit/scorer.test.ts

@@ -87,7 +87,7 @@ describe('scoreAnomaly', () => {
       fc.property(
         fc.float({ min: 0, max: 100, noNaN: true }),
         fc.float({ min: 0, max: 100, noNaN: true }),
-        fc.float({ min: 0.01, max: 50, noNaN: true }),
+        fc.float({ min: Math.fround(0.01), max: 50, noNaN: true }),
         (costA, costB, stddev) => {
           const baseline = { mean: 5.0, stddev };
           const scoreA = scoreAnomaly({ cost: Math.min(costA, costB), ...baseline });