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>

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