test: add unit tests for postprocess blending functions

[wangwangbobo]

Closes #414

Added 4 new tests for issue #414:

1. test_laplacian_blend_identity - blending image with itself returns same image
2. test_laplacian_blend_shapes - output shape matches input for various resolutions (64x64 to 1024x768)
3. test_feathered_mask_range - output pixel values stay in [0, 255] range
4. test_histogram_match_preserves_structure - SSIM correlation > 0.85

All tests use synthetic numpy arrays — no GPU or model weights needed.

[dreamlessx]

Welcome @wangwangbobo! Great first contribution — these 4 tests directly address #414 and are well-structured:

• Identity blend test — correct use of tolerance for pyramid numerical error
• Shape tests — good coverage across resolutions (64×64 through 1024×768)
• Feathered mask range — properly tests with gradient mask
• Histogram preservation — correlation check is a smart lightweight alternative to full SSIM

CI needs to run — I'll approve the workflow. Once it passes, this is ready to merge.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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