feat(deck): add Moonlight handoff preflight contract

[papi-ux]

Summary

• Adds a local-only Steam Deck Moonlight handoff preflight contract.
• Blocks runtime/network/process/host mutation boundaries while still producing reviewable typed argv preview data.
• Adds C++ coverage plus a static source guard so this lane cannot quietly grow launch/network behavior.

Verification

• RED captured: focused preflight test failed before production files existed (stream/deck_moonlight_handoff_preflight.h missing).
• cmake -S clients/deck -B build/deck-final-core -DNOVA_DECK_BUILD_QT_SHELL=OFF
• cmake --build build/deck-final-core -- -j$(nproc)
• ctest --test-dir build/deck-final-core --output-on-failure → 12/12 passed
• cmake -S clients/deck -B build/deck-final-qt
• cmake --build build/deck-final-qt -- -j$(nproc)
• ctest --test-dir build/deck-final-qt --output-on-failure → 13/13 passed

Notes

• This deliberately does not resurrect stale QML/main bridge churn from the old branch. Current master already has the richer backend preflight DTO surface; this PR keeps the Moonlight handoff contract narrow and reviewable.