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>

1 files changed, 14 insertions, 0 deletions

diff --git a/src/lib/config.validate.source.sh b/src/lib/config.validate.source.sh
index 28aeace..e3c55e7 100644
--- a/src/lib/config.validate.source.sh
+++ b/src/lib/config.validate.source.sh
@@ -32,6 +32,19 @@ validate_optional_positive_integer_config_var() {
     fi
 }
 
+# A percentage config var: an integer in the inclusive range 0..100. Unlike
+# validate_positive_integer_config_var, 0 is allowed (it is the natural "off"
+# value, e.g. THUMB_SUBDIVIDE_PERCENT=0 disables tile subdivision entirely).
+validate_percentage_config_var() {
+    local -r name="$1"; shift
+    local -r value="${!name}"
+
+    if [[ ! "$value" =~ ^[0-9]+$ ]] || (( value > 100 )); then
+        config_error "$name must be an integer between 0 and 100"
+        return 1
+    fi
+}
+
 validate_yes_no_config_var() {
     local -r name="$1"; shift
     local -r value="${!name}"
@@ -273,6 +286,7 @@ validate_common_config() {
     validate_optional_positive_integer_config_var HEIGHT || return
     validate_positive_integer_config_var THUMBHEIGHT || return
     validate_positive_integer_config_var MAXPREVIEWS || return
+    validate_percentage_config_var THUMB_SUBDIVIDE_PERCENT || return
     validate_positive_integer_config_var IMAGE_JOBS || return
     validate_positive_integer_config_var IMAGEMAGICK_TIMEOUT || return
     validate_positive_integer_config_var TAR_TIMEOUT || return