Fix running TUI in various sbt modes

[jatcwang]

Summary

• run the viewer directly from ordinary sbt 1 shells while preserving thin-client client jobs
• force a forked runner under sbt 2 to avoid JLine classpath conflicts
• use JLine’s controlling-terminal fallback so forked viewers inherit full terminal dimensions

Testing

• sbt --client "++2.12.21 ; sbtPlugin / scripted difflicious/mixed-fork-nonfork-modules"
• sbt --client "++3.8.4 ; sbtPlugin / scripted difflicious/mixed-fork-nonfork-modules"
• sbt --client "cli3 / test"
• manually verified diffliciousViewer with the sbt 1 client, sbt 1 shell, and sbt 2 shell, including terminal resizing

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