Add configurable spread neighborhood radius (SpreadRad)

[DLWoodruff]

Notice

This PR adresses a well-known deficiency in Cell2Fire: because of the small spread neighborhood, the fire scares look very long and thin under high wind.
This PR was mostly the work of codex under my supervision. I have tested this change very lightly on Cell2Fire
so someone with an active model should try larger neighborhoods.

Summary

• add --SpreadRad (default 1) to configure fire spread neighborhood radius
• keep legacy behavior for radius 1 and support larger Moore neighborhoods for n > 1
• refactor spread propagation bookkeeping to avoid angle-key collisions for larger neighborhoods
• retain the source-cell ROS assumption during outgoing spread updates
• add CI integration test script (test/spreadrad_test.sh) and run it in build-debian-stable.yml
• document SpreadRad in README.md

Validation

• local shell syntax check for new test script passed (bash -n test/spreadrad_test.sh)
• A full local build could not be completed on my computer due missing local tiffio.h; upstream CI workflow installs required dependencies and will validate build/test execution.
• tests were successfully run on Cell2Fire
• more validation is needed