Fix signed/unsigned mismatch in DpcProcessor::ComputeResultImages

[Copilot]

Addresses feedback from PR #21 regarding unnecessary type conversion in nGIProcessor.cpp that created a signed/unsigned mismatch.

Changes

• Changed N from ptrdiff_t back to size_t to match openbeamDC.Size() return type
• Moved cast to loop comparison only: for (ptrdiff_t i=0; i<static_cast<ptrdiff_t>(N); i++)

Before:

const ptrdiff_t N=static_cast<ptrdiff_t>(openbeamDC.Size());
#pragma omp parallel for
for (ptrdiff_t i=0; i<N; i++)

After:

const size_t N=openbeamDC.Size();
#pragma omp parallel for
for (ptrdiff_t i=0; i<static_cast<ptrdiff_t>(N); i++)

This preserves the natural unsigned type while maintaining OpenMP compatibility where ptrdiff_t is required for the loop variable.

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