fix: harden FFmpeg/NVENC startup and Windows binary runtime paths

[zeter1]

Fixes the FFmpeg 9.0.2 startup failure reported from the packaged EXE logs and hardens adjacent failure handling.

Changes:

• probe NVENC encoder options with ffmpeg -h encoder=... and only emit supported optional AQ flags;
• keep the corrected -spatial-aq / -temporal-aq syntax;
• only fall back from ddagrab to gdigrab for proven Desktop Duplication startup errors;
• preserve the original FFmpeg/NVENC error and stderr for non-capture failures;
• update diagnostic summary/report when recording fails before the first frame;
• keep settings/logs/default output out of temporary scoped_dir launch folders;
• add regression coverage for option syntax, optional-flag omission, fallback classification, and temp-path handling.

Expected verification: Windows CI, verify_project.py, compileall, packaged PyInstaller smoke build.