Keep 2x2 features off the page bottom; flush short final pages

Two layout-breaking cases remained after the fixed-column grid landed:

1. A 2x2 feature spans two rows, so one placed near the bottom of a page
   left an L-shaped gap grid-auto-flow: dense could not backfill (nothing
   follows it) -- a cut-off corner. append_preview_grid now only offers the
   feature layout while at least feature_tail_margin (16) photos remain, so
   a hero always sits in the upper rows with enough trailing 1-cell tiles to
   complete its rows at every breakpoint (and short pages get no hero).

2. A short final page (a leftover handful of photos) could be subdivided
   down below 12 cells, where it can't be aligned to a multiple of 12 and is
   ragged. Such pages now go through _build_final_page_tiles: plain singles,
   then merge down to a multiple of 12 (count >= 12 -> flush grid) or a
   full-row "fill" filmstrip (count < 12 -> one clean banner, or stacked
   banners). Longer/full pages keep the normal roll-and-align path.

Verified hole-free at 2/3/4/6 columns with a grid-auto-flow:dense packing
simulator across ~480 generated pages (regular + short final), 0 failures.

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

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