Implement BFS and a Job Scheduler

The interview report described two coding questions in the same category: 1. **Grid reveal with BFS** You are given an `m x n` board representing a hidden minefield. Each cell is one of: - `M`: unrevealed mine - `E`: unrevealed empty cell - `B`: revealed blank cell with no adjacent mines - `1`-`8`: revealed cell showing the number of adjacent mines - `X`: revealed mine after a click Given a click position `(row, col)`, update the board using these rules: - If the clicked cell is a mine (`M`), change it to `X`. - Otherwise, reveal cells using **BFS**: - Count the number of mines in the 8 neighboring cells. - If the count is greater than 0, write that digit in the cell and stop expanding from it. - If the count is 0, mark the cell as `B` and continue revealing its unrevealed neighbors. Return the updated board. 2. **Concurrent periodic job scheduler** Implement a thread-safe in-memory scheduler that supports periodic jobs. Each job has: - a unique `jobId` - an execution interval such as every `N` milliseconds - a callback function to run Support the following operations: - `register(jobId, intervalMs, task)` - `cancel(jobId)` - `start()` - `stop()` Requirements: - Multiple worker threads may execute jobs concurrently. - A job should be triggered approximately once per interval. - The same scheduled run must not execute more than once. - Cancellation must be safe even if other threads are polling or executing jobs. - Explain how you would handle cases where a task runs longer than its interval, clock drift, and backpressure under heavy load.