feat(deck): show Moonlight readiness checklist preview

[papi-ux]

Summary

• Add a preview-only Moonlight handoff readiness checklist to the Nova Deck right rail.
• Surface safe static gates for host snapshot, app snapshot, typed argv review, and focus-return review.
• Keep the slice non-executing: it does not launch Moonlight, Sunshine, games, streams, host discovery, pairing, networking, or persistence.

Verification

• CMake configure/build for Nova Deck debug target
• CTest Deck suite
• Moonlight handoff source guard
• git diff --check
• Qt offscreen --smoke-exit
• Private staged Steam Deck/Game Mode capture verified the 1280x800 Nova Deck window and readable readiness chips

Non-goals / safety boundary

• No Moonlight/Sunshine invocation
• No game or stream launch
• No host discovery, pairing, HostStore, credentials, or runtime persistence
• No release/deploy behavior