Fix job race condition and guard exit callbacks

[aphroteus]

When initiating a new search while a previous job is running, stdout and exit callbacks from superseded jobs continue to execute because 's:id' is overwritten, polluting the quickfix list.

Additionally, if tabs or windows are closed during an async search, restoring focus using static window numbers throws errors or focuses the wrong window.

• Assign a monotonic job_id to each task and discard callbacks from superseded jobs
• Use win_gotoid() for precise window/tab restoration with bounds-checked fallback