Add dynamic subdivided thumbnail tiles

Make the album preview grid livelier: with a configurable probability
(THUMB_SUBDIVIDE_PERCENT, default 30%) a square thumbnail tile is
subdivided into several smaller thumbnails packed into the same square
footprint, chosen at random from:

- quad: 2x2 squares (4 photos)
- two-wide: two stacked full-width strips (2 photos)
- squares+wide: two squares plus one full-width strip, strip on the
  top or the bottom (3 photos)

Each sub-thumbnail stays its own clickable photo with its own view page;
subdivision only groups consecutive photos visually, so preview
numbering and the view/details/redirect pages are unchanged. No new
images are generated (CSS object-fit crops the existing aspect-correct
thumbs into squares or wide strips). Sub-thumbnails get the same random
entry animation and the same dramatic hover (flip/scale/rotate/filter)
as full thumbs. The layout choice reuses the seeded random_index, so
builds stay reproducible under RANDOM_SEED. THUMB_SUBDIVIDE_PERCENT=0
reproduces the previous output byte-for-byte.

The new option is wired through the config defaults, validation
(0..100), --print-config, the --subdivide CLI flag, the usage text, and
the shuriken.json / --dry-run metadata, with docs and tests updated.
Generated HTML and CSS pass the W3C Nu HTML checker and CSS validator.

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

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