goldstein.py: Fix empty bottom right edges

[scottstanie]

1. Fixed edge handling - Changed from range(0, shape - psize, step) to proper padding that covers all pixels
2. Reflection padding on all sides - Instead of zero-padding (which causes artifacts), uses np.pad(..., mode="reflect") to maintain signal continuity at edges
3. Weight normalization - Added weight_sum accumulator to properly normalize the output, especially important at edges where fewer windows overlap
4. Fixed empty_mask - Changed from np.angle(data) == 0 (which incorrectly treats valid phase=0 as empty) to data == 0 (only treats zero-magnitude complex values as empty)

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• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review