Add full FD order support (2nd/4th/6th/8th) to C derivative kernels via ghost_width dispatch
Wrap each C kernel in #if (ghost_width == N) blocks matching Fortran stencil coefficients from diff_new.f90, kodiss.f90, and lopsidediff.f90. Add fast-path indexing for ord=1,4,5 in share_func.h. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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