Fix the Target, Precompute Once: A Backend-Free Word-Ladder Solver With a BFS Distance Field

When you build an interactive puzzle, the latency budget is unforgiving. Every keystroke needs an answer that feels instant. A daily word-ladder game has to do three of those instant jobs at once: confirm that the word a player typed is legal, tell them the best possible score for the day, and, on request, reveal the shortest solution.