fix: keep blame metadata aligned while scrolling

Blame metadata could drift away from source lines when the first visible commit group started above the viewport. Large groups could also clip their hash at the top or make adjacent labels overlap, while measuring only visible lines could change the margin width during scrolling.

Align metadata with AvaloniaEdit line centers, protect the first group only when there is enough room, and measure all blame data consistently so scrolling keeps labels stable and readable.

Author: gadfly3173