Fix embedding robustness issues and add CI regression guards

[clemsgrs]

Summary

• fix 4 runtime robustness issues in the embedding pipeline (main.py, embed.py, aggregate.py, models.py)
• add deterministic regression guard tests in test/test_regression_bugfixes.py
• wire those regression guards into GitHub CI via a new regression-guards job in .github/workflows/pr-test.yaml
• keep existing Docker integration test, now dependent on regression guards

Bug fixes included

1. Region model factory: ensure region-level branches assign tile_encoder before RegionFeatureExtractor wrapping.
2. Safe process group termination: launch subprocesses with start_new_session=True before killpg use.
3. Distributed deadlock reduction: remove per-rank barrier() calls from inside the per-slide try block and propagate failures consistently.
4. Safe output-dir handling: avoid Path(cfg.output_dir, None) when --output-dir is not provided.

Validation

• python3 -m unittest discover -s test -p 'test_regression_bugfixes.py'
• python3 -m compileall slide2vec/main.py slide2vec/embed.py slide2vec/aggregate.py slide2vec/models/models.py

Notes

• slide2vec/hs2p submodule code was not edited.