fix(wayland): keep HDR metadata off mapping anchor

[Lowband21]

Summary

• retain the non-null 1x1 host buffer required for recursive subsurface mapping on KWin
• keep that transparent host unscaled and untagged instead of applying child video HDR metadata to it
• delegate the actual video surface image description to GStreamer 1.28+ waylandsink
• tone-map to SDR when native nested-surface color management is unavailable
• preserve synchronized, untagged PGS subtitle composition

This follows up #4: mapping the intermediary host made its existing manual HDR tag participate in composition, reintroducing green artifacts when SDR PGS overlays changed.

Testing

• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• cargo check -p ferrex-player --features mpv against this local Subwave checkout