Organize bash libraries with per-module docs and tests

[codeforester]

fix: correct several bugs in lib_std.sh

• Fix color gate logic: color is "0"/"1" so -z check was always false, causing colors to always enable in interactive terminals regardless of the --color flag
• Add COLOR_BOLD to the readonly declaration (was set but not protected)
• Add missing >&2 redirect on unsupported OSTYPE error message
• Fix wait_for_enter to actually wait for Enter by removing -n1
• Fix assert_integer_range error message to include the tested value and use clearer range notation
• Remove spurious "$@" argument passed to dump_trace (ignored but wrong)
• Remove redundant >&2 on log_debug call (already writes to stderr)
• Fix Usaage typo in safe_mkdir comment