Computes aspect at generate-time from the loaded BufferedImage: w/h
above 1.1 → LANDSCAPE, otherwise PORTRAIT. The threshold keeps
near-square A4 scans in the portrait tile (ratio ≈ 1.0) rather than
flipping to landscape on a rounding error. Also hardens the pipeline
with an explicit dimension guard so width=0 / height=0 edge cases fail
cleanly instead of dividing by zero when the aspect is computed.
Refs #305
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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