fix(qt): embed Windows NVST presenter as child HWND

[zortos293]

Corrects the Windows presentation contract introduced by #785.

• Reparent the out-of-process SDL presenter with SetParent, convert it to a borderless WS_CHILD, and position it with Qt-host-local physical geometry.
• Preserve focus and Raw Input ownership through the Qt parent HWND, while suspending keyboard, mouse, and gamepad forwarding whenever the presenter is hidden or the session is paused.
• Make overlay opening wait for the hide/input-release acknowledgement, and make Qt surface teardown wait for checked presenter detachment.
• Report Windows honestly as embedded-child; this remains an out-of-process child HWND, not a Qt Quick scene-graph texture. Other platforms remain paired auxiliary windows.

Verification:

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo check --workspace
• cargo test --workspace
• cargo check -p opennow-streamer-platform --target x86_64-pc-windows-gnu with LLVM-MinGW 18
• Full Qt CMake build and ctest (60/60 passed)
• git diff --check

Real-Windows runtime acceptance is still required to prove taskbar/top-level behavior, DPI/display transitions, fullscreen, overlay hide/restore, native input, reconnect, and teardown against an active NVST stream.

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