Fix signed/unsigned type mismatch in DpcProcessor::ComputeResultImages

[Copilot]

Addresses feedback on PR #21 regarding incorrect type conversion in nGIProcessor.cpp line 281. The variable N was changed from size_t to ptrdiff_t with an explicit cast, creating an unnecessary signed/unsigned mismatch with the return type of openbeamDC.Size().

Changes

• Reverted N to const size_t to match Size() return type
• Removed unnecessary static_cast<ptrdiff_t> from variable initialization
• Added explicit cast in OpenMP loop condition: i<static_cast<ptrdiff_t>(N)

// 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 approach keeps the type conversion localized to where OpenMP requires ptrdiff_t for the loop variable, consistent with existing patterns in the codebase (line 233).

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