Update README with LeetCode Daily Challenge

Author: jeremymanning

README.md

@@ -479,6 +479,7 @@ _**Note:** Our group is currently taking a break from grinding leetcode. The tab
 | October 9, 2025 | [3494](https://leetcode.com/problems/find-the-minimum-amount-of-time-to-brew-potions/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3494) | 🟡 Medium |
 | October 10, 2025 | [3147](https://leetcode.com/problems/taking-maximum-energy-from-the-mystic-dungeon/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3147) | 🟡 Medium |
 | October 11, 2025 | [3186](https://leetcode.com/problems/maximum-total-damage-with-spell-casting/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3186) | 🟡 Medium |
+| October 12, 2025 | [3539](https://leetcode.com/problems/find-sum-of-array-product-of-magical-sequences/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/3539) | 🔴 Hard |
 
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