refactor: move write options to write/options.rs

[Its-Just-Nans]

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[amazon-q-developer]

This is a clean refactoring PR that successfully moves the write options code from src/write.rs to a new src/write/options.rs module. The changes are well-organized and all imports have been properly updated across the codebase.

The refactoring correctly:

• Moves FileOptions, ExtendedFileOptions, FullFileOptions, SimpleFileOptions, EncryptWith, and related code to src/write/options.rs
• Updates all import statements throughout the codebase to use the new module paths
• Moves ffi constants from types.rs to format/ffi.rs for better organization
• Consolidates Magic imports to use format::magic::Magic consistently
• Adds comprehensive tests to src/format/flags.rs for the System enum

No defects or issues were identified that would block merging. The code compiles correctly and maintains all existing functionality.

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