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>

3 files changed, 7 insertions, 4 deletions

diff --git a/docs/configuration.md b/docs/configuration.md
index 5d61a27..69a8835 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -14,6 +14,7 @@ values for the current run.
 | `THUMBHEIGHT` | `300` | Thumbnail height in pixels. Positive integer. |
 | `MAXPREVIEWS` | `40` | Maximum previews per page. Positive integer. |
 | `THUMB_SUBDIVIDE_PERCENT` | `30` | Percent chance (0-100) that a preview tile is subdivided into smaller thumbnails (2x2 quad, two stacked wide strips, or two squares plus one wide strip on top/bottom). Each sub-thumbnail is its own clickable photo. `0` disables it. |
+| `THUMB_FEATURE_PERCENT` | `10` | Percent chance (0-100) that a preview tile is a large "feature" tile: a single photo spanning a 2x2 block of the overview grid. Rolled before the subdivision chance. `0` disables it. |
 | `IMAGE_JOBS` | `3` | Parallel jobs for image processing and HTML template rendering. Positive integer. |
 | `IMAGEMAGICK_TIMEOUT` | `60` | Per-ImageMagick-command timeout in seconds. Positive integer. |
 | `TAR_TIMEOUT` | `120` | Tarball creation timeout in seconds. Positive integer. |
@@ -54,7 +55,8 @@ The checks (details in `src/lib/config.validate.source.sh`):
   `IMAGE_JOBS`, `INCOMING_DIR`, `DIST_DIR`, `TEMPLATE_DIR`.
 * **Positive integers**: `THUMBHEIGHT`, `MAXPREVIEWS`, `IMAGE_JOBS`,
   `IMAGEMAGICK_TIMEOUT`, `TAR_TIMEOUT`; `HEIGHT` is an optional positive integer.
-* **Percentage (0-100 integer)**: `THUMB_SUBDIVIDE_PERCENT`.
+* **Percentage (0-100 integer)**: `THUMB_SUBDIVIDE_PERCENT`,
+  `THUMB_FEATURE_PERCENT`.
 * **`yes`/`no` settings**: `SHUFFLE`, `SPLASH_PAGE`, `STATS_PAGE`,
   `TARBALL_INCLUDE`, `SYNC_DELETE` (where applicable).
 * **Readable input**: `INCOMING_DIR` must be a readable directory; `TEMPLATE_DIR`
@@ -77,7 +79,7 @@ Generation stops before writing album output when validation fails.
 
 `CONFIG_SOURCE`, `INCOMING_DIR`, `DIST_DIR`, `TEMPLATE_DIR`, `FAVICON`,
 `SOURCE_URL`, `TITLE`, `HEIGHT`, `THUMBHEIGHT`, `MAXPREVIEWS`,
-`THUMB_SUBDIVIDE_PERCENT`, `IMAGE_JOBS`,
+`THUMB_SUBDIVIDE_PERCENT`, `THUMB_FEATURE_PERCENT`, `IMAGE_JOBS`,
 `IMAGEMAGICK_TIMEOUT`, `RANDOM_SEED`, `SHUFFLE`, `SPLASH_PAGE`, `STATS_PAGE`,
 `TARBALL_INCLUDE`, `TARBALL_SUFFIX`, `TAR_TIMEOUT`, `TAR_OPTS`, `SYNC_DELETE`,
 `SYNC_DESTINATIONS`, `ORIGINAL_BASEPATH`.
diff --git a/docs/generation.md b/docs/generation.md
index b89cafc..4d29840 100644
--- a/docs/generation.md
+++ b/docs/generation.md
@@ -97,8 +97,8 @@ metadata records:
 * generated photo, thumbnail, and HTML file counts;
 * tarball status (included + file);
 * effective settings (title, height, thumbheight, maxpreviews, subdivide
-  percent, image jobs, random seed, shuffle, splash page, stats page, original
-  basepath) useful for debugging a published album.
+  percent, feature percent, image jobs, random seed, shuffle, splash page, stats
+  page, original basepath) useful for debugging a published album.
 
 ## Favicon
 
diff --git a/docs/usage.md b/docs/usage.md
index f4c8d27..3f85471 100644
--- a/docs/usage.md
+++ b/docs/usage.md
@@ -69,6 +69,7 @@ config variable documented in [configuration.md](configuration.md).
 | `--thumbheight VALUE` | `THUMBHEIGHT` |
 | `--maxpreviews N` | `MAXPREVIEWS` |
 | `--subdivide PERCENT` | `THUMB_SUBDIVIDE_PERCENT` |
+| `--feature PERCENT` | `THUMB_FEATURE_PERCENT` |
 | `--image-jobs N` | `IMAGE_JOBS` |
 | `--random-seed VALUE` | `RANDOM_SEED` |
 | `--shuffle` | `SHUFFLE=yes` |