Replace _split_oversized_rows() with _regularize_row_grid(). When ≥60%
of content rows have consistent height (±25% of median), overlay a
uniform grid with the standard row height over the entire content area.
This leverages the fact that books/vocab lists use constant row heights.
Validates grid by checking ≥85% of words land in a grid row. Falls back
to gap-based rows if heights are too irregular or words don't fit.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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