Support FreeBSD and macOS via resolved GNU tool variables

shuriken relied on GNU-only extensions (find -printf, stat -c, cp -a,
sort -R) and hard-refused to run outside Linux. Resolve each of the four
tools once at startup to a FIND/STAT/CP/SORT variable, preferring a
g-prefixed sibling (gfind, gstat, gcp, gsort) when present on PATH -
mirroring the $SED/$GREP/$DATE tool-selection pattern in the sibling
gemtexter project - so the same source runs unmodified on Linux, macOS,
and FreeBSD once GNU coreutils/findutils are installed there.

- src/lib/compat.source.sh: add resolve_gnu_tool + FIND/STAT/CP/SORT;
  add a fast verify_gnu_tool_versions preflight (--version contains
  "GNU") ahead of the existing behavioral probes, which now run against
  the resolved variables instead of hardcoded tool names; split the
  probes and error reporting into their own ~30-line functions.
- Replace the GNU-only find -printf / stat -c / cp -a / sort -R call
  sites in photo-list, image, album-photo-select, metadata-cache,
  config.staging, and random source files with the resolved variables.
  POSIX-portable find/sort calls elsewhere are untouched.
- README.md: drop the "Linux-only" claim; document brew/pkg GNU
  coreutils+findutils install steps for macOS and FreeBSD.
- tests/cli.sh: add coverage for the g-prefixed-sibling preference and
  for the new --version-string preflight's error message, alongside the
  existing behavioral-probe guard tests.

just build / just check-generated / just shellcheck / just test /
git diff --check all pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/lib/photo-list.source.sh b/src/lib/photo-list.source.sh
index 9e7ce72..754e09e 100644
--- a/src/lib/photo-list.source.sh
+++ b/src/lib/photo-list.source.sh
@@ -23,11 +23,13 @@
 # suppressed (the stats background loader) adds its own 2>/dev/null. This is the
 # SORTED listing; album_photo_files deliberately keeps its own maybe_shuffle
 # variant because the album's display order is the configurable shuffle, not a
-# plain sort.
+# plain sort. Uses $FIND (compat.source.sh): -printf is GNU-only, so this must
+# run against the resolved GNU find, not whatever "find" plain resolves to on
+# macOS/FreeBSD. The trailing plain "sort" is POSIX-portable and left as-is.
 list_photos() {
     local -r photos_dir="$1"; shift
 
-    find "$DIST_DIR/$photos_dir" -maxdepth 1 -type f -printf '%f\n' | sort
+    "$FIND" "$DIST_DIR/$photos_dir" -maxdepth 1 -type f -printf '%f\n' | sort
 }
 
 # Pick one seeded-random entry from an already-collected list, addressed by