Passthrough.Connector: forward-ported v17 into v18

Passthrough.Connector: forward-ported repo/CI scaffolding from v17/main into v18/main (LICENSE, README, CHANGELOG, CI workflows, Directory.Build.props, etc.) — v18/main had none of it yet
Passthrough.Connector: regenerated passthrough-client/package-lock.json and Jumoo.TranslationManager.Passthrough/packages.lock.json to resolve a merge conflict and fix a stale NuGet lock pinned to 17.0.0 that broke CI's locked-mode restore
Passthrough.Connector: opened, fixed, and merged Jumoo/Passthrough.Connector#6; cleaned up the merge branch afterward