Book-like reading the user explicitly wanted: pairIntoRows groups pages into
two-up spread rows (optional coverAlone for a title page; trailing odd page sits
alone), spreadRowHeights fits each page to half the column and takes the tallest
per row (common baseline), and spreadStackMetrics stacks the rows so the
existing PageStackMetrics.visibleRange windows continuous-DOUBLE by ROW.
Pure geometry reusing the continuous-single layer; the row-mounting widget is
device-gated. Zero-rework-risk (not rendering).
flutter analyze lib/editor clean; 172/172 tests (+8).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
