Compute nearest bathroom distance for each desk

You are given an `m x n` grid of characters containing only: - `'B'` = bathroom - `'D'` = desk - `'_'` = empty cell From any cell you may move one step up/down/left/right (no diagonals). The distance between two cells is the length of the shortest such path. For every desk cell `'D'`, compute the distance to its nearest bathroom `'B'`. Return an `m x n` integer matrix where: - for a desk cell, the value is the nearest-bathroom distance, - for non-desk cells, you may return `-1` (or any clearly documented placeholder). If a desk cannot reach any bathroom, return `-1` for that desk.