ci: benchmark CircleCI M4 Pro Large

[kyleve]

Posted by an AI agent on kyleve's behalf.

Summary

• temporarily run both CircleCI iOS jobs on m4pro.large
• hold the Xcode image, commands, caches, simulator, and test configuration constant
• compare the resulting wall time and credit cost with the merged m4pro.medium baseline from ci: migrate iOS tests to CircleCI M4 Pro #237

Why

CircleCI's Large worker doubles the managed macOS allocation from 6 CPUs / 28 GB to 12 CPUs / 56 GB, while also doubling compute price from 200 to 400 credits per minute. This PR isolates that resource-class change so its CI run can show whether the added parallelism improves elapsed time enough to justify the cost.

Review focus

This is a temporary benchmark PR and changes only the shared CircleCI executor's resource class. It is not intended to merge until the measured timing and cost trade-off are reviewed.

Testing

• circleci config validate .circleci/config.yml
• circleci config process .circleci/config.yml
• git diff --check