* Add virtual display for headless Depot runners
Depot macOS runners have no physical display, causing XCUITests to fail
with "Failed to activate application (current state: Running Background)".
This adds a small ObjC tool that creates a virtual display using the
private CGVirtualDisplay API before tests run.
* Split self-hosted concurrency groups per workflow
CI, nightly, and release all shared `self-hosted-build`, so the hourly
nightly cancelled in-progress CI runs. Now each workflow has its own
group (self-hosted-ci, self-hosted-nightly, self-hosted-release).
CI also gets cancel-in-progress: true so rapid pushes cancel stale runs.
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