Add threaded GDAL harness for GeoTIFF and ERS I/O

[Codex]

Designed a Python harness to exercise GDAL reads/writes in a multithreaded flow across GeoTIFF and ERS rasters.

• Harness: Added raster_threading_harness.py to generate a deterministic grid, write GTiff/ERS concurrently, then read both in parallel and report stats.
• Docs: Updated README.md with prerequisites and usage flags for customizing threads, size, and output directory.

Example invocation:

python raster_threading_harness.py --output-dir /tmp/raster-artifacts --threads 4 --size 256

Original prompt

Design a test harness to use gdal via python to read and write to rasters in multi-threaded fashion. Use both geotiffs and ers grids in your work.