fix: forward frame stepping position sync

[United600]

StepForwardOneFrame now checks for position changes after advancing a frame and raises the PositionChanged event if the time has changed. Close #787.

[Copilot]

Pull request overview

Fixes forward frame-stepping position desync by synchronizing _position and raising PositionChanged when stepping forward changes playback time.

Changes:

• After NextFrame(), reads VlcPlayer.Time and converts it to a TimeSpan
• Updates _position when changed and raises PositionChanged with old/new values

[Copilot]

This introduces an inline _position update + PositionChanged raise pattern. If this class already has a single place that mutates _position (e.g., a Position setter, SetPosition(...), or a helper that raises PositionChanged), consider routing this through that shared path to keep event semantics consistent and prevent future drift between different position-update call sites.

Suggested change

	var oldValue = _position;
	_position = newValue;
	PositionChanged?.Invoke(this, new ValueChangedEventArgs<TimeSpan>(newValue, oldValue));
	Position = newValue;