Implement Sync for ThreadsafeFitsFile

[simonrw]

Summary

• Only Send was manually implemented for ThreadsafeFitsFile
• Since the inner Mutex ensures exclusive access, Sync is also safe
• Without Sync, &ThreadsafeFitsFile cannot be shared between threads (e.g. stored in Arc for shared references or used with rayon parallel iterators)

Test plan

• All existing tests pass (including the 10,000-thread stress test)

🤖 Generated with Claude Code

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/threadsafe-sync-impl

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.5%. Comparing base (83b76a5) to head (61eac02).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
fitsio/src/threadsafe_fitsfile.rs	100.0% <ø> (ø)

... and 2 files with indirect coverage changes

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