Adopt async subprocess orchestration

[flaticols]

Summary

• adopt swift-subprocess 0.5 and move container orchestration from blocking Process.waitUntilExit() calls to cancellable async subprocesses
• replace blocking health-check sleeps with Task.sleep, run independent health checks concurrently, and avoid polling a dependency again after it passes a gate
• remove process-global CONTAINER_CLI mutation from tests by allowing explicit executable injection
• stream updater package downloads to temporary files and run pkgutil/installer through the async runner
• bump ComposeKit from 0.0.3 to 0.0.4

Compatibility

• source builds now require Swift 6.2 or newer
• public ContainerComposeKit process/orchestration methods are now async
• CLI command behavior and syntax remain unchanged

Validation

• swift build -Xswiftc -warn-concurrency -Xswiftc -strict-concurrency=complete
• swift test --parallel — 39 tests pass
• swift build -c release
• release binary --help smoke test
• quickstart up --dry-run smoke test