Word Break

Write `word_break(s, word_dict)` returning `True` when the string can be split into a sequence of dictionary words. Words may be reused. word_break("leetcode", ["leet","code"]) -> True Greedy longest-match fails: on "aaaaab" with ["aaaa","aaa","b"] taking the longest first strands the rest. Keep a boolean per prefix — position i is reachable if some word ends there starting from a reachable position.