Add cleanup path unregister support

[codeforester]

Summary

• Add public std_unregister_cleanup_path with the same safe-path validation model as cleanup registration.
• Unregister std timeout markers and file-section temp paths after eager cleanup or successful atomic moves.
• Cover cleanup registry shrinkage in std and file BATS tests.

Fixes #149

Validation

• bats lib/bash/std/tests/lib_std.bats
• bats lib/bash/file/tests/lib_file.bats
• git diff --check origin/main...HEAD
• ./tests/validate.sh