[Feature]: Show SwiftUI debug-build distance from its upstream base

[saphid]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case.

Area

SwiftUI client in apps/swift-ios on the owner branch from #5178; build tooling.

Problem or use case

A locally installed SwiftUI Debug build identifies itself as a development build, but it does not show whether its source is current with the branch it was built from. When testing a fork or retained worktree, it is easy to mistake an older build for the latest upstream state.

Proposed solution

At install time, derive a public base ref and embed the ahead/behind commit counts in the Debug build metadata. Extend the existing development-build badge to show that distance. Allow an explicit base-ref override and otherwise use the repository upstream default branch, with no private host, signing, or release data.

Why this matters

Testers can immediately tell whether an installed development build is stale or contains local commits, reducing invalid bug reports and repeated reinstalls.

Smallest useful scope

Debug builds only. Extend the existing Workspace development badge, Info.plist, and Scripts/install-device.sh. Do not change Release/TestFlight identity or signing behavior.

Alternatives considered

Showing only the Git commit identifies a build but does not answer whether it is ahead or behind its intended base. A settings-only presentation is less visible during active testing.

Risks or tradeoffs

Base-ref discovery must be deterministic and optional. Missing Git metadata should degrade to the existing badge rather than blocking a build. Visible proof should cover ahead, behind, equal, and unavailable metadata.

References

SwiftUI owner PR: #5178.

Contribution

• I would be open to helping implement this.