[file-diet] Split FfmpegVideoRecorder into focused partial files

[Copilot]

FfmpegVideoRecorder.cs had grown to 723 lines, mixing lifecycle orchestration, ffmpeg process handling, and Windows window-capture interop. This change refactors the type into focused files with partial classes while preserving behavior and internal API usage.

• File-diet refactor

  • Moved VideoSegment into VideoSegment.cs as a standalone data model.
  • Converted FfmpegVideoRecorder to partial and split concerns across files.

• Lifecycle/API surface kept in main file

  • FfmpegVideoRecorder.cs now holds core fields/properties and lifecycle orchestration (Start, StopAsync, PruneSegments, ConcatAsync).
  • Main file reduced to <300 lines for easier navigation.

• Process management extracted

  • Added FfmpegVideoRecorder.ProcessManagement.cs for ffmpeg invocation/output handling and argument construction:
    • RunFfmpegAsync, WaitForExitAsync, TryKill, DetachAndDispose, OnFfmpegOutput, ClearRecentFfmpegOutput, FindFfmpeg, BuildSegmentArguments, BuildDefaultInput
    • Segment parsing/diagnostics methods remain on the same type and are callable from the main partial.

• Windows capture/interop isolated

  • Added FfmpegVideoRecorder.WindowCapture.cs for DPI-aware window region detection and P/Invoke:
    • TryGetCurrentProcessWindowRegion, TrySetPerMonitorDpiAwareThread, EnumerateCandidateWindows, nested NativeMethods

// Main file
internal sealed partial class FfmpegVideoRecorder { /* lifecycle */ }

// Process/file split
internal sealed partial class FfmpegVideoRecorder
{
    private string BuildDefaultInput() { /* ... */ }
}

// Windows-specific split
internal sealed partial class FfmpegVideoRecorder
{
    private static bool TryGetCurrentProcessWindowRegion(out int x, out int y, out int width, out int height) { /* ... */ }
}