Improve error message for Windows multiprocessing memory/pagefile failures

[phflot]

When multiprocessing workers fail to start due to low virtual memory (e.g., "paging file is too small"), pyflowreg currently surfaces BrokenProcessPool without actionable guidance. Please catch/translate this case and suggest reducing buffer_size/n_jobs or switching to threading/sequential.

[phflot]

This requires a complete fallback strategy for low memory / too many workers e.g.:

1. Catch BrokenProcessPool in BatchMotionCorrector._process_batch_parallel (src/pyflowreg/motion_correction/compensate_recording.py).
2. Retry same batch once with fewer multiprocessing workers (n -> n//2, min 1).
3. If retry fails, fallback to threading, then sequential.
4. Emit warnings so behavior is transparent.
5. Optional next layer: preflight memory-based worker cap before launching pool.