Package for PyPI publication

Nobody has claimed this yet.

Packaging SST for publication and installation via PyPI has been expressed as a desirable update to the project in a few contexts.

The first decision that would be needed for this though is a package name because bare [sst](https://pypi.org/project/sst/) is claimed.

And changes that seem appropriate:
- Drop GroundingDINO (or excise it into a branch for further exploration of text-based segmentation, perhaps for evaluation alongside SAM3)
- Migrate local SAM2 copy use to [sam2](https://pypi.org/project/sam2/) imports and default configs
- Specify `from sst import` to avoid ambiguous imports
- Retrieve model weights from HF automatically rather than as a manual step
- Set entry points and a simple `cli.py` module, with launching the GUI as its own subcommand
- Pin down dependencies a bit
- PyPI Actions workflow
- README and demo updates for `data/`
- Housekeeping DOI, Zenodo, etc.

This should be most of what's needed for packaging. Scope here should probably be limited to just what's needed for PyPI publication.

This would obviously be a breaking change for anyone currently using the scripts from the cloned repo. We should probably want to tag a release for the current 'scripts era' at v1.1.0 and release the updated PyPI as v2.0.0.

Any/all of the above is available for discussion!

Start by deciding the package name and narrowing the listed changes to what PyPI publication requires. Review the proposed cli.py entry point, README and data/ updates, dependency and model-weight handling, and the PyPI Actions workflow. Done means the project can be installed from PyPI with documented entry points and a tagged release.

Written by the indexing model from the issue text.