Use async-friendly Process execution in SPMBuildSystem

[obj-p]

SPMBuildSystem.runProcess() and Compiler.run() both use Process.waitUntilExit() which blocks the actor's cooperative thread. This is pre-existing in Compiler but more impactful in SPMBuildSystem where swift build can run for seconds.

Replace with process.terminationHandler + withCheckedContinuation to avoid blocking cooperative threads during long builds.

Low priority — the current approach works fine in practice since actor re-entrancy isn't an issue for these sequential build operations.

Identified in PR #3 review.