PY-PORT-2b: port tarball_shrink_report.py to aprender-ci-tools #4788

Sub-ticket of #4741 (the Python-to-Rust port of the build helpers).

Port scripts/lib/tarball_shrink_report.py to a tarball-shrink-report subcommand of crates/aprender-ci-tools.

Acceptance:

• scripts/tests/ci_tools_py_parity_test.sh runs the same cases through the original (read from git at its last commit, as the external validator) and the Rust bin. stdout is byte-identical and the exit code agrees, covering: empty inputs, a full fixture, missing inputs (exit 2), one bad input (a directory named x.rs, exit 1 on both), and the live crates/ tree.
• A planted defect in the port turns the table red.
• The only caller, scripts/package_tarball_build.sh, runs the subcommand. scripts/lib/tarball_shrink_report.py is deleted.
• The deliberate differences (a usage error exits 1 under clap, not 2; Unicode combining marks count as word characters) are listed in the crate README.