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>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/shuriken.default.conf b/src/shuriken.default.conf
index 737eb26..28da780 100644
--- a/src/shuriken.default.conf
+++ b/src/shuriken.default.conf
@@ -12,6 +12,11 @@ MAXPREVIEWS=40
 # strip with the strip on top or bottom). Each sub-thumbnail is still its own
 # clickable photo. 0 disables subdivision; 100 always subdivides.
 THUMB_SUBDIVIDE_PERCENT=30
+# Percent chance (0-100) that a thumbnail tile is a large "feature" tile: a
+# single photo blown up to span a 2x2 block of the overview grid. Rolled before
+# the subdivision chance. 0 disables feature tiles; 100 makes every tile a
+# feature.
+THUMB_FEATURE_PERCENT=10
 # Parallel jobs for image processing and HTML template rendering.
 IMAGE_JOBS=3
 # Timeout in seconds for each ImageMagick command.