feat(deck): add no-network stream media groundwork

[papi-ux]

Summary

Add the Steam Deck stream/media groundwork needed for future native streaming without starting any live network/runtime behavior yet.

This PR introduces a no-network stream session seam around moonlight-common-c callback structures, Linux media capability/probe modeling for FFmpeg VAAPI and PipeWire/Pulse dependency state, focused CTests, and updated Deck-native port study notes.

Scope included

• clients/deck/CMakeLists.txt: Deck core build wiring, moonlight-common-c linkage, Linux media pkg-config probes, and stream tests.
• clients/deck/README.md: no-network stream skeleton boundary notes.
• clients/deck/spikes/streaming-backend-notes.md: native backend decision notes.
• clients/deck/src/stream/deck_stream_core.*: inert stream session lifecycle and callback adapter groundwork with network start disabled.
• clients/deck/src/stream/deck_stream_media_adapters.*: Linux media adapter capability/probe model.
• clients/deck/tests/deck_stream_core_test.cpp and deck_stream_media_adapters_test.cpp.
• docs/steam_deck_native_port_study.md: updated native-port decision notes.

Explicitly not included

• Deck UI/QML polish.
• Game Mode capture harness scripts/tests.
• Live Moonlight/Sunshine invocation.
• LiStartConnection runtime startup.
• Host discovery/pairing or HostStore persistence.
• Credentials, certificates, or session-token writes.
• Polaris live endpoints or live streaming behavior.

Validation

• git diff --check origin/master...HEAD -- clients/deck docs/steam_deck_native_port_study.md
• cmake --build build/deck-stream-media-pr-b
• ctest --test-dir build/deck-stream-media-pr-b --output-on-failure -R 'nova_deck_stream_(core|media_adapters)_test'
• ctest --test-dir build/deck-stream-media-pr-b --output-on-failure

Safety notes

The added stream session remains inert and no-network. It initializes callback structs and state transitions only; networkStartAllowed remains false and tests verify the native streaming start path is not called.