When a JSON line wraps to many visual rows (e.g. 73 rows), pressing j
would skip the entire logical line, making wrapped content unreadable.
Now j/k scroll by 1 visual row when a VisualHeightIndex is available,
with cursor tracking the viewport center. Falls back to logical-line
scroll in Loading/no-index modes or when all content fits the viewport.
Adds 4 tests: visual scroll down, visual scroll up, small-file fallback,
j/k roundtrip.
8c5a838db0