Flush preview-grid rows: fixed breakpoint columns + multiple-of-12 pages

The overview grid used auto-fill columns (an unpredictable count at view
time) while tiles per page were fixed at generation, so the last row was
ragged -- an empty, cut-off bottom-right corner, made worse by a 2x2
feature tile.

CSS (header.tmpl): replace auto-fill with a FIXED column count per width
breakpoint -- 2 (phone) / 3 / 4 / 6 -- all divisors of 12. THUMBHEIGHT no
longer drives the grid (it only sizes the thumbnail files), so its
obsolete render-var is dropped (template.source.sh).

Generator (album-tile-layout, album-thumbnail-html): append_preview_grid
now decides a page's tiles, then snaps the grid-cell total onto a multiple
of 12 before emitting, via two photo-preserving levers --
_grid_split_subdivides_to_add (round up: split subdivided tiles into
singles; preferred, abundant) and _grid_merge_singles_to_remove (round
down: merge adjacent singles). Because 2/3/4/6 all divide 12, a
multiple-of-12 page tiles into a COMPLETE rectangle at every breakpoint:
a flush last row at any window width, with no image distortion
(object-fit: cover). Per-photo preview numbers and all navigation
redirects are unchanged. Tiny pages (a short final page or small stats
mini-album) are left as-is.

Decrements use assignment, not bare "(( --k ))": under set -euo pipefail
an arithmetic command evaluating to 0 returns status 1 and would abort
generate. Helper namerefs are uniquely named and arrays are forwarded by
name to avoid bash circular-nameref errors.

Tests: add test_album_grid_cells_align_to_multiple_of_12 (both levers);
update the two tests that pinned the old auto-fill CSS / render-var list.

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

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