Clarify cleanup path batch registration

[codeforester]

Summary

• Register safe cleanup paths from mixed batches while rejecting unsafe paths.
• Return nonzero when any cleanup path in the batch is rejected.
• Document and test the mixed-batch cleanup contract.

Validation

• shellcheck --severity=error lib/bash/std/lib_std.sh
• bats lib/bash/std/tests/lib_std.bats
• ./tests/validate.sh

Note: shellcheck --severity=error lib/bash/std/tests/lib_std.bats currently reports an unrelated pre-existing SC2314 warning outside this change.

Fixes #82