feat(deck): add Moonlight handoff preflight preview

[papi-ux]

Summary

• Adds a local-only Deck Moonlight handoff preflight model with explicit non-runtime safety gates.
• Surfaces the preflight in Nova Deck as a visible review panel: Moonlight handoff preview — Nothing will launch yet.
• Extends tests/source guards so the model, Qt bridge, QML panel, and CMake wiring stay preview-only.

Safety boundaries

This PR does not add runtime streaming or launch behavior:

• No Moonlight/Sunshine/Steam/game launch.
• No host discovery, pairing, HostStore, or persisted credentials.
• No live Polaris/Nova runtime calls.
• No network launch or process execution path.
• Preflight runtime flags remain closed: executable=false, allowsNetwork=false, allowsProcessExecution=false, allowsMoonlight=false, allowsHostMutation=false.
• QML rendering gates preview details on safeToRender plus all runtime gates being closed.

Test plan

• Public repo surface: bash scripts/check-public-surface.sh
• Linux Qt build: cmake --build clients/deck/build-preflight-qt -j$(nproc)
• Linux CTest: 7/7 passed
• SteamOS container build using ianburgwin/steamos:base-devel
• SteamOS container CTest: 7/7 passed
• Real Steam Deck staged smoke: nova-deck --smoke-exit rc 0
• Real Steam Deck Game Mode capture rc 0
• Capture selected the Nova Deck / nova-deck window at 1280x800
• Exact process-name cleanup check found no lingering nova-deck, moonlight, Moonlight, or sunshine processes
• git diff --check HEAD~2..HEAD

Notes

• The right-side safe-launch card is intentionally dense but readable at 1280x800; follow-up polish can make that area scrollable or split the details cleaner.
• This is still a preflight/review UI slice, not the real Moonlight runtime handoff.