ci: run tests on Blacksmith macOS 15, not GitHub-hosted
A GitHub billing block must never stop CI. ci.yml tests moves from macos-15 to blacksmith-6vcpu-macos-15. The job still runs xcode-select -switch /Applications/Xcode_26.0.1.app; this PR's own run proves the Blacksmith macOS 15 image carries that Xcode (an earlier finding showed a Blacksmith macOS 26 image missing an Xcode pin). If it fails there, the fix is to select an Xcode that the image has, not to go back to GitHub-hosted.
No guard workflow is added, to keep the fork small.
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Summary by cubic
Moves CI tests from GitHub-hosted macos-15 to blacksmith-6vcpu-macos-15 so a GitHub billing block can never stop CI. The job still selects Xcode 26.0.1, and this PR's run confirms the Blacksmith image includes it. If that install ever fails, the fix is to pick an Xcode the image has rather than returning to GitHub-hosted runners. No guard workflow is added to keep the fork small.
Summary by CodeRabbit
- Chores
- Updated the macOS environment used for automated test runs. This is an internal maintenance change and does not alter app features or behavior.