Add runtime GNU-tool guard; document Linux-only platform support

shuriken shells out to GNU-only features of the standard Unix tools
(find -printf, stat -c, cp -a, sort -R). Add require_gnu_tools in a new
src/lib/compat.source.sh, sourced early and invoked from main() before any
action runs. On invocation it feature-probes each tool in a throwaway temp
dir; if any probe fails it prints a clear error naming the offending tool
and exits 1, so non-GNU (macOS/BSD) environments fail fast instead of
producing broken output.

README gains a Platform compatibility section and the requirements line now
mentions GNU coreutils/findutils. Tests cover the find and stat rejection
paths; the shared test helper that builds a coreutils-without-imagemagick
PATH now includes cp and stat (which the guard probes).

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