hex: cursor follows the scroll (stays visible on wheel/scrollbar)

Override watch_scroll_y so a viewport scroll drags the byte cursor along: when
the rounded scroll changes, clamp the cursor's row into the visible range
[top, top+height) keeping its column, so it rides the nearest edge instead of
being left off-screen. No feedback loop -- key-nav's _scroll_to_cursor already
puts the cursor in view (clamp is then a no-op), and HexView.Moved only updates
the status line. Pairs with the click-to-place support.

Pilot `hex` scenario updated: after a scroll the cursor must now be within the
visible rows (was: asserted it stayed put). 9/9 hex checks pass.

0 files changed, 0 insertions, 0 deletions