Add packaged data access helpers

[SuperdoerTrav]

Summary

Repository Policy Checklist

• Changes preserve the existing file layout (ssapy_toolkit/, tests/, demos/, docs/, scripts/, .github/).
• Package-code changes include tests under tests/, or this PR only changes docs/CI/metadata.
• New user-facing workflows include a runnable demo under demos/, or no demo is needed because:
• No generated outputs, downloaded data, images, notebooks with embedded outputs, binary media, or large artifacts are committed.
• Persistent data needed by this work is stored in SSAPy-Data or documented as an external input.

Validation

• pytest tests
• python -m ssapy_toolkit.run_all_demos --no-open when demos or user-facing workflows changed
• python scripts/check_repository_policy.py