cask: avoid outputting Rosetta 2 caveats on Intel machines

Built-in caveats with runtime conditions (requires_rosetta,
files_in_usr_local) were pre-rendered into the JSON API caveats string
at serialization time. On Intel Macs, the Rosetta 2 warning was shown
because the arch guard never ran for API-sourced installs.

This separates conditional built-in caveats from the pre-rendered string:
- Strips requires_rosetta text from the serialized caveats string
- Adds a caveats_rosetta boolean field to the cask JSON
- Re-evaluates the Rosetta caveat at install time based on current arch

On Intel Macs, the Rosetta 2 caveat is no longer shown. On ARM Macs
without Rosetta installed, it continues to appear.

Fixes #18365.

This contribution was developed with AI assistance (Claude Code + Codex).