Viewer: present frames through a DXGI flip-model swap chain

[chammoru]

Summary

• replace Viewer HWND GDI presentation with a D3D11/DXGI flip-model swap chain while preserving the existing GDI composition and overlays in a top-down BGRA DIB
• retain the verified Viewer: possible horizontal tearing during video playback after back-buffer reuse #88 double-DWM-wait path as an automatic fallback and as a developer-selectable path through Q1VIEW_DISABLE_DXGI
• decode unrotated, unprocessed OpenCV BGR video directly into playback buffers to remove an extra full-frame copy
• add opt-in Q1VIEW_TRACE_PLAYBACK timing diagnostics for backend, displayed/dropped frames, paint time, and decoder time

Root cause

The existing playback paint path waited for DWM twice per displayed frame. On a 60 Hz desktop this limited the representative 59.94 fps video to about 26–30 displayed fps. Moving completed frames to flip-model swap-chain buffers makes presentation atomic and removes those two waits from the normal path.

Validation

• C0610_main.mp4 (Sony ZV-1F, 1920×1080 HEVC Main, 60000/1001 fps, 11,700 frames): 11,675 displayed follow-up frames, 24 dropped (0.21%), 195.197 s playback, approximately 59.81 displayed fps; GDI fallback comparison was approximately 26.3 fps
• exact paused-frame pixel stability, resume, one-frame right/left stepping, synchronized playback between two Viewer processes, EOF, resize, minimize/restore, and full-screen transitions verified through actual Viewer UI automation
• generated 642×360 59.94 fps H.264 long-GOP video: all 179 follow-up frames displayed with zero drops, including a non-DWORD-aligned source width
• Core regression tests passed
• Viewer Release x64 full rebuild: 0 warnings, 0 errors
• Comparator Release x64 build: 0 warnings, 0 errors

Outstanding manual acceptance

The original #88 tall/portrait seam must still be checked on the affected company 4K display before this PR is marked ready or merged. The Actions artifact from this draft PR is intended for that test.

Closes #90 only after the outstanding 4K acceptance test passes.