For some text decorations (e.g. underline) we were using the line's top
edge as the Y-coordinate to draw the line at, which combined with the
line's thickness meant that it was positioned too high up.
Correct this by calculating the line's center Y position.
Replace per-frame heap-allocated RefCounted ScrollFrame objects with a
single contiguous Vector<ScrollFrame> inside ScrollState. All frames for
a viewport are now stored in one allocation, using type-safe
ScrollFrameIndex instead of RefPtr pointers.
This reduces allocation churn, improves cache locality, and moves
parent-chain traversal (cumulative offset, nearest scrolling ancestor)
into ScrollState — similar to how visual context nodes were recently
consolidated into AccumulatedVisualContextTree.