Add large 2x2 feature tiles and a CSS-grid overview

Allow a single photo to be blown up into a large "feature" tile that
spans a 2x2 block of the album overview, controlled by a new
THUMB_FEATURE_PERCENT (0-100, default 10; 0 disables). Each tile rolls
for a feature first, then for a subdivision, otherwise stays a normal
square.

To pack mixed-size tiles (normal 1x1, subdivided 1x1, feature 2x2)
without gaps, the overview is now a real CSS grid with
grid-auto-flow: dense, so smaller tiles backfill the holes a 2x2 feature
would leave. Tile spacing moved from per-image padding to the grid gap.
Feature tiles reuse the img.thumb class and its dramatic hover.

THUMB_FEATURE_PERCENT is wired through the same layers as
THUMB_SUBDIVIDE_PERCENT: config defaults, 0..100 validation,
--print-config, the --feature 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.
Setting both percentages to 0 reproduces the previous all-1x1 grid.

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

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