Add atomic evidence write regression tests

[tayfuryldz]

Context

Evidence writers rely on atomic_write_text() so metadata/export files are not left half-written. The helper sits on a data-integrity boundary and currently lacks a focused regression contract.

Scope

Add deterministic tests for src/headerproof/file_safety.py using tmp_path and monkeypatching only.

Acceptance criteria

• Parent directories are created when needed.
• Replacing an existing destination yields complete new content.
• The temporary sibling is not left behind after a successful write.
• An fsync OSError is tolerated as intended and the destination is still replaced.
• Tests do not weaken atomic replacement semantics.
• Existing tests remain green.

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