fix: include tied embeddings in selective downloads

[burak33bb]

Summary

• include tie_word_embeddings from normalized config when selecting weight files to download for the final shard
• add regression coverage for tied embeddings where embed_tokens must be downloaded for the last shard

Duplicate check

• reviewed contribution guide and recent open/closed issues and PRs
• searched for tie_word_embeddings selective download lm_head, selective download, and tied embeddings download; only the original selective-download feature PR appeared, with no duplicate fix

Tests

• python -m black --check src/parallax/utils/model_download.py tests/test_model_download.py
• python -m ruff check --select E,F src/parallax/utils/model_download.py tests/test_model_download.py
• direct invocation of tests/test_model_download.py test functions passed with PYTHONPATH=src

Note: pytest tests/test_model_download.py could not collect on this Windows environment because repo tests/conftest.py imports mlx.core and MLX fails to load its DLL here. Full editable install also hit uvloop does not support Windows.