refactor(qt): make NVST presentation shell-owned

[zortos293]

Summary

• adds a typed Qt StreamSurfaceController that owns stream-host discovery, stable handle/geometry updates, DPR-aware physical presentation bounds, logical capture bounds, deduplication, reconnect replay, overlay visibility, and stale-handle-free teardown
• makes Qt/QML own fullscreen, stream menus, suspended-video stats, input handoff, recovery, and errors while keeping the out-of-process NVST presenter responsible only for video, audio, native input, cursor, and pointer lock
• normalizes every legacy transport setting to explicit NVST, removes the obsolete core browser-signaling path, fails microphone upstream closed, and retains NVIDIA-required ICE/DTLS/SCTP control behavior
• removes native stats/debug/chrome rendering and its font dependency, forwards legacy fullscreen/stats shortcuts to Qt exactly once, and publishes bounded live FPS/bitrate telemetry for the QML stats surface
• documents the current paired auxiliary-window limitation honestly on Windows, X11, Wayland, and macOS; ordinary QML never claims to composite over live native video, and real cross-platform hardware acceptance remains required

This is stacked on #781 because it is a focused follow-up to the Qt rebuild branch.

Validation

• cmake --build /home/user/.capy/work/opennow-qt-build -j2
• ctest --test-dir /home/user/.capy/work/opennow-qt-build --output-on-failure — 60/60 passed
• cargo fmt --all -- --check, cargo clippy --locked --all-targets -- -D warnings, and cargo test --locked --all-targets in native/opennow-core
• cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, cargo check --workspace, and cargo test --workspace in native/opennow-streamer
• npm --prefix opennow-stable run locales:check — 891 used keys checked
• git diff --check

A Linux-hosted macOS cross-check was blocked before project compilation because the environment has no Apple C toolchain/SDK (cc rejected -arch and -mmacosx-version-min). The macOS CI runners and required real-device acceptance remain the authoritative checks for that path.

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