Add negotiated bandwidth-saving quality policy

[zortos293]

Summary

Adds a default-off Save bandwidth control to the Qt desktop and console settings. New sessions request NVIDIA's prefer-FPS dynamic quality policy. The core preserves the policy in the negotiated session profile, and NVST ANNOUNCE uses that profile rather than a mutable preference. Resumed sessions keep their original policy.

The change also adds a real FFmpeg regression for a midstream resolution change. Existing decoder, format-change and Vulkan image-reallocation paths remain the presentation owners.

Independent verification

• cargo test --manifest-path native/opennow-core/Cargo.toml --locked: 420 passed, 4 ignored.
• cargo test --manifest-path native/opennow-streamer/Cargo.toml --workspace --locked --features linux-ffmpeg-bundled: 681 passed, 16 ignored.
• Explicitly ran the normally ignored real-codec test with the same workspace command plus ffmpeg_software_decoder_reports_a_midstream_resolution_change -- --ignored. It passed with real H.264 packets at 256x144 and 320x180.
• Locked strict Clippy, formatting, 158 Python contract tests and npm run locales:check passed.
• Debug Qt build and 5 focused settings tests passed. ctest --test-dir build/opennow-qt --output-on-failure passed 342/342.
• The frozen 12-file tree matched the tested checkpoint byte-for-byte before publication.

No live vendor session, GPU adaptation measurement or bandwidth-reduction benchmark was performed. The setting requests the server policy; it does not promise a particular reduction.

Rendered acceptance fixture

Dependency

Base: capy/enable-capability-gated-360-fps at b470c330c59d411ad18eb03b5a629d0f8e920896. Land #968 and its parents first, then revalidate the integrated stack.

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[capy-ai]

Resolved the actual integration conflicts in normal merge 57eaff9a, preserving store-launch and bandwidth localization, resources, and smoke workload routes. The merged feature diff remains limited to the published bandwidth slice; the separate network-test layer is not pulled in early.

Independent verification passed 574 core tests with 4 ignored and 773 bundled-FFmpeg native tests with 16 ignored, strict locked Clippy in both workspaces, and formatting. The worker rebuilt Qt, passed all 349 Qt tests, reran the real midstream-resolution-change fixture, and inspected the rendered bandwidth setting. Required cross-platform CI is now running on the updated head.