Refactored calc_derivs and calc_dderivs to include correct boundary
handling logic matching the legacy code. Implemented fallback to 2nd
order derivatives when near boundaries where 4th order stencils cannot
be used. Added logic to initialize output arrays to zero to avoid
uninitialized memory access.
- Implemented cache blocking (BLK=8) in bssn_rhs_opt.f90 to improve L1/L2 cache hit rate.
- Introduced bssn_rhs_opt.f90 module with vectorized derivative and physics kernels.
- Renamed original implementation to bssn_rhs_legacy.f90 for fallback.
- Updated bssn_rhs.f90 to act as a dispatcher, using the optimized path for ghost_width=3.
- Updated makefile to include new source files.
- Added DEBUG_NAN_CHECK macro to optionally disable NaN checks in production.
