Add an opt-in SteamVR launch mode

[oliverw]

Parent

#21

What to build

Add an explicit SteamVR mode to dcs-linux launch. A user with the supported SteamVR prerequisites can select VR while keeping the same prepared prefix, mapped game directory and saved games, pinned runtime, install targeting, and process-tree ownership as a normal launch.

The command applies only the configuration proven by #22. Flat-screen launch remains the default and is unchanged.

Acceptance criteria

• dcs-linux launch --vr appears in help and accepts the existing install selector
• VR launch uses the SteamVR/OpenXR configuration and startup order proven by Prove DCS Standalone through SteamVR on Linux #22
• Launch refuses clearly when the selected install or required SteamVR runtime state is unavailable instead of silently starting flat-screen
• The game directory and saved games remain outside the disposable prefix
• Any game-file change follows the existing Integrity Check risk gate; IC-safe environment, arguments, or prefix changes are preferred
• Interrupting or ending the command owns and cleans up the complete umu, Proton, Wine, DCS process tree without taking ownership of an already-running SteamVR session
• Running dcs-linux launch without --vr behaves exactly as before
• Text, JSON, exit-status, prerequisite-failure, and flat-screen regression behaviour are covered by tests and user documentation

Blocked by

• Prove DCS Standalone through SteamVR on Linux #22 — Prove DCS Standalone through SteamVR on Linux