Replace RepeatColumns stub with real implementation: compacts a test column
left against column 1 to derive the copy distance, tiles further columns at
that interval, and clips partial columns to the work area bounds. Adds 4 new
FillExtentsTests covering multi-column fill, rect shapes, non-zero-origin
work areas, and cancellation.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Replace AdjustColumn stub with a convergence loop that distributes the
remaining gap between the topmost part and the work area top edge across
all pairs. TryAdjustPair/TryShiftDirection try shifting part2 up (or down
as fallback) and compact left, rejecting moves that widen the pair.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
