[BugFix] Stabilize cross-format checkpoint recovery

[vmoens]

Fixes cross-format checkpoint recovery when two manifests have identical creation timestamps, as observed on Windows. Rotation now uses filesystem modification nanoseconds before filename ordering, so interrupted replacements keep the newly written checkpoint regardless of container format.

The existing interrupted-replacement test now forces equal creation times and distinct write order.

Validation:

• test/test_checkpoint.py: 60 passed, 1 skipped
• pinned ufmt check
• pinned flake8 check

Observed in: https://github.com/pytorch/rl/actions/runs/34016957895/job/101452018472

[pytorch-bot]

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