Add configurable spread neighborhood radius (SpreadRad)

[DLWoodruff]

Summary

• add configurable spread neighborhood radius via --SpreadRad (default 1)
• refactor C++ spread neighbor handling to support multiple neighbors sharing the same direction
• retain existing spread assumption: outgoing spread from source cell uses source-cell ROS
• add regression tests for SpreadRad=1 compatibility and SpreadRad=2 behavior
• add dedicated GitHub Actions workflow for spread-radius tests
• add high-level docs page and link it from index.rst

Notes

• SpreadRad controls spread neighborhood geometry
• IgnitionRad remains separate and unchanged in intent

Validation

• local C++ build succeeded
• local tests/test_spread_radius.py passed