prebuild performance improvements

[jpdriver]

Description

Warning

(changes generated by GPT-5)

• evals "platforms" Array in app.json
  • if you only have "platforms": ["android"] we skip anything to do with iOS
  • if you explicitly specify a platform like ./scripts/prebuild.sh --platform ios it has to be included in app.json or the script will error
• additionally, we now only do bundle install if we don't already have a /vendor directory
  • the Ruby Gems are unlikely to change frequently, so this speeds up consecutive ./scripts/prebuild.sh runs
  • note that /vendor is wiped out by ./scripts/init.sh so Gems will now only be re-installed when doing a full ./scripts/init.sh run

GIF

[Scure21]

CI is failing with unrelated UI tests, going to merge anyways, we can check the tests in another PR.