Recover corrupted checkpoint downloads

[kiwigitops]

Summary

• download checkpoints to a temporary file before replacing the final .ckpt
• redownload an existing checkpoint once when torch.load hits a truncated archive error
• use the same checkpoint-loading path for the CLI and wrapper APIs

Refs #11

Tests

• $env:PYTHONPATH='src/simplefold'; python -m pytest tests/test_checkpoint_utils.py -q
• python -m compileall -q src/simplefold/utils/checkpoint_utils.py src/simplefold/inference.py src/simplefold/wrapper.py tests/test_checkpoint_utils.py
• git diff --check

[kiwigitops]

Hi, checking in on this when you have a chance. The change is scoped to making checkpoint downloads atomic and retrying once when an existing .ckpt is detected as truncated, so interrupted downloads do not leave a permanently bad cache file.

[kiwigitops]

Quick follow-up here. The atomic checkpoint download/retry path is still current, and the checkpoint utility tests passed on my side. Happy to adjust if needed.