Fix deterministic checkpoint file sharding

[koriyoshi2041]

Problem

split_checkpoint_files() partitions the raw result of os.listdir(). Directory enumeration order is not guaranteed, so ranks on different nodes can derive different file-to-rank assignments for the same checkpoint. That can make checkpoint registration overlap some shards and omit others.

Fix

Sort the filtered .safetensors paths before slicing them across ranks.

The regression test presents the same directory contents in forward and reverse enumeration order and verifies that both produce the same complete, non-overlapping assignment.

Tests

uv run pytest -q tests/test_example_update.py
1 passed

uv run ruff check examples/update.py tests/test_example_update.py
All checks passed!

uv run ruff format --check examples/update.py tests/test_example_update.py
2 files already formatted