feat(play-setup): add per-game encoder selection and truthful HUD health

[papi-ux]

Summary

• add a per-game encoder selector to Play Setup with Host default, Auto, and host-advertised named backends
• persist the choice through normal launches and pinned shortcuts
• carry the encoder request and launch assertions through the host contract
• keep healthy SHM sessions and clean Auto Safe bitrate recovery out of ATTN
• retain actionable warnings for confirmed media loss and authoritative amber/red Doctor findings

Behavior

• Host default preserves the host configuration
• Auto allows a live per-session probe and fallback
• named encoders are required for that launch and never silently fall back
• older hosts remain compatible when the new capability fields are absent
• healthy recovery is displayed as muted context rather than a warning

End-to-end encoder selection depends on papi-ux/polaris#598.

Verification

• full testNonRoot_gameDebugUnitTest suite passes
• assembleNonRoot_gameBenchmark passes
• cross-repository contract audit reports no new drift
• signed benchmark APK installed over the existing RP6 app with application data, pairing, and the 24-game library preserved
• Play Setup persisted strict NVIDIA NVENC for a real title
• the launch reached 1920x1080 at 120 FPS over HEVC with CUDA selected and no fallback
• healthy control retries remained green; confirmed media loss still raised a critical state and cleared when the evidence cleared
• abrupt disconnect caused no host restart or coredump