Add directory listing sort option

[gnm3000]

This adds a new --sort CLI option for directory listings. The default behavior remains unchanged with name-based sorting, and users can now choose date-based sorting with newest entries first.

Supported values:

• name
• modified / mtime
• created / birthtime

The option is wired through the CLI, server options, core option parsing, and directory listing renderer. Documentation and tests were added for the new behavior.

Relevant issues

N/A

Contributor checklist

• Provide tests for the changes (unless documentation-only)
• Documented any new features, CLI switches, etc. (if applicable)
  • Server --help output
  • README.md
  • doc/http-server.1 (use the same format as other entries)
• The pull request is being made against the master branch

Maintainer checklist

• Assign a version triage tag
• Approve tests if applicable