[sharmis-tp]

Summary

• Extracted a shared write_atomic(path, write_fn) helper in src/serialize/atomic.rs
• save_tensors (fdl) and save_safetensors now write to {path}.partial.{pid}, sync_all(), then rename — a crash mid-write never destroys the existing checkpoint
• Temp file is cleaned up on write error so it never accumulates
• Added 4 robustness tests: truncated fdl/safetensors detection, temp cleanup, and round-trip through the atomic path

Closes #13

Test plan

• cargo test — all 186 tests pass (including 4 new robustness tests)
• Truncated fdl checkpoint does not destroy the good copy
• Truncated safetensors does not destroy the good copy
• Atomic write cleans up temp on fdl save error
• save_tensors round-trips through atomic write