Validate VM file descriptors before ownership transfer

[fallintoplace]

Summary

• Reject negative --vm-fd values during CLI parsing.
• Duplicate the supplied descriptor with F_DUPFD_CLOEXEC before taking ownership.
• Verify that the duplicate is an open AF_UNIX datagram socket.
• Add coverage for negative, closed, non-socket, non-datagram, and ownership cases.

Why

VM::new previously converted the caller-supplied raw descriptor directly into a UnixDatagram. Invalid input could fail unpredictably, and dropping the VM wrapper could close a descriptor still owned by the caller. Softnet now owns only a validated duplicate.

Validation

• cargo fmt --all -- --check
• cargo check --all-targets
• cargo check --target aarch64-apple-darwin --all-targets
• Focused library and CLI tests passed directly.

The repository's full test command is configured to run through sudo -E; that runner could not authenticate in this environment, so the focused test binaries were run without the configured runner.

[chatgpt-codex-connector]

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[fkorotkov-oai]

LGTM. Could you please check linting issues