Check if all substrings are dictionary words

You are given a string `s` (letters only) and access to an English dictionary `dict` (a set of valid words). Return `true` if **every contiguous substring** of `s` with length **>= 3** is a valid dictionary word (i.e., for all `i, j` with `0 <= i <= j < n` and `j - i + 1 >= 3`, `s[i..j] ∈ dict`). Otherwise return `false`. Clarifications: - Substring means **contiguous**. - You may assume dictionary lookups are available (e.g., `dict.contains(word)`), or you can preprocess the dictionary. Example: - If `s = "cats"`, you must check `"cat"`, `"ats"`, and `"cats"` (and any other substrings of length >= 3).