Use platform-specific cache paths and respect XDG_CACHE_HOME

[liby]

Description:

Use proper platform-specific methods to determine cache directory paths:

1. First check XDG_CACHE_HOME environment variable

2. On macOS:

   • Use NSFileManager.URLForDirectory to get system cache path (~/Library/Caches)

   • Use bundle ID (com.mitchellh.ghostty) as base directory to follow macOS conventions

3. Fall back to XDG spec defaults for other platforms (~/.cache)

Changes:

• Extract common NSFileManager path lookup logic into makeCommonPath helper

• Use NSFileManager.URLForDirectory to get proper macOS cache directory

• Use bundle ID for macOS cache directory to match system conventions and appSupportDir behavior

  • Follows the same pattern as our configuration file path (~/Library/Application Support/com.mitchellh.ghostty/config)

  • Aligns with appSupportDir implementation which already uses bundle ID

• Add tests to verify path construction for different platforms

Questions for reviewers:

1. Should we add migration logic for existing cache files?

2. Or should we document this as a breaking change and let users manually clean up?

Testing:

1. Added unit tests for path construction:

   • macOS: verifies ~/Library/Caches/com.mitchellh.ghostty path

   • Linux: verifies ~/.cache/ghostty path (XDG spec)

2. Verified tests pass with zig build test

Fixes #3202

[jparise]

I think this is generally a good change and addresses a blocker for some people on macOS, so I'm motivated to see a solution get merged. Thanks for working on this!

There are some follow-ups that I think are interesting to think about afterward, but I don't think any of them are blocking.

[vahur]

Not really able to review the changes as I don't know the codebase. As long ghostty starts with read-only home directory and uses platform directories, it's enough for me atm. Parity with Application Support, bundle id wise, would be nice but not a show stopper.

[mitchellh]

I don't think xdg.cache should return anything but the XDG cache dir as specified.

I think the calling point for xdg.cache should be modified to call this for macOS instead.

[mitchellh]

Thanks, this looks good. I noted one issue/question.