Send container machine run forced-exit diagnostic to stderr

[vyncint]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Part of #642.

container machine run installs a SignalThreshold handler to report the forced exit after three SIGINT/SIGTERMs, but emits that diagnostic with print(), so it goes to stdout.

The identical handler in the sibling commands uses log.warning(), which the CLI routes to stderr through StderrLogHandler:

• ContainerRun.swift#L167
• ContainerExec.swift#L105

The handler is only installed when !tty — precisely when stdout is likely a pipe or a file — so the diagnostic is interleaved into the machine process's redirected output rather than kept on the terminal.

This routes the message to stderr, matching the sibling commands. let log = self.log is bound before the closure for the same reason it is in those two: SignalThreshold.start takes a @Sendable @escaping closure.

Testing

• Tested locally

• Added/updated tests

• Added/updated docs

• swift build -c debug -Xswiftc -warnings-as-errors -Xswiftc -enable-testing — clean

• make test — 613 tests in 71 suites passed

• swift format lint --strict --configuration .swift-format-nolint on the changed file — clean

• Confirmed the stream routing with the built binary: logger output goes to stderr (container image load -i /nonexistent writes only to stderr) and print() output goes to stdout (container --version).

I did not exercise the three-signal path end to end, as that needs a booted container machine; the change is a like-for-like swap to the logger the sibling commands already use on that same code path.