feat: implement APPE (append) command per RFC 959

[benedikt-schesch]

Summary

Implements the FTP APPE (append) command as defined in RFC 959. This command appends data to an existing file, or creates a new file if it doesn't exist.

Changes

• Add Appe variant to Command enum and parser
• Add Appe variant to DataChanCmd for data channel communication
• Implement Appe command handler in commands/appe.rs
• Implement exec_appe() in datachan.rs - gets current file size via metadata() and calls put() with that offset
• Add integration tests for:
  • Appending to an existing file
  • Appending to a non-existent file (creates it)

RFC 959 Reference

This command causes the server-DTP to accept the data transferred via the data connection and to store the data in a file at the server site. If the file specified in the pathname exists at the server site, the data shall be appended to that file; otherwise the file shall be created.

Testing

cargo test appe

[benedikt-schesch]

#52

[benedikt-schesch]

Full disclosure: kiro-cli and Claude Opus 4.5 have been used in this PR but I verified the diff to make sure it makes sense

[benedikt-schesch]

@hannesdejager Hi Hannes, any chance you could spare this a review? :) Thanks