Merge plotting optimizations from chb-copilot-test

- Implement multiprocessing-based parallel plotting
- Add parallel_plot_helper.py for concurrent plot task execution
- Use matplotlib 'Agg' backend for multiprocessing safety
- Set OMP_NUM_THREADS=1 to prevent BLAS thread explosion
- Use subprocess for binary data plots to avoid thread conflicts
- Add fork bomb protection in main program

This merge only includes plotting improvements and excludes
MPI communication changes to preserve existing optimizations.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

plot_GW_strain_amplitude_xiaoqu.py

@@ -11,6 +11,8 @@
 import numpy                               ## numpy for array operations
 import scipy                               ## scipy for interpolation and signal processing
 import math
+import matplotlib
+matplotlib.use('Agg')                      ## use non-interactive backend for multiprocessing safety
 import matplotlib.pyplot    as     plt     ## matplotlib for plotting
 import os                                  ## os for system/file operations