Assert generated page tiles flush (cell total multiple of 12)

Closes the last output-coverage gap from the gauge-bar audit: the flush
guarantee (a page's grid-cell total is a multiple of 12 so it forms a
complete rectangle at 2/3/4/6 columns) was only unit-tested on the
alignment helper and via markup presence. The dense-packing check that
verified it on real albums was a throwaway. test_generate_preview_grid_
emits_every_tile_shape now computes the generated page-1 cell total
(features=4 cells, others=1) and asserts it is a positive multiple of 12,
and that the pinned seed still yields both a feature and a subdivided tile.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions