hex: freeze the cursor's screen position on scroll (not clamp-to-edge)

Replace the clamp-to-nearest-edge follow with a true screen-position freeze:
watch_scroll_y now shifts the byte cursor by the exact scroll delta on a user
scroll (wheel/scrollbar), so it points at a new byte but stays on the same screen
row. Cursor-driven scrolls (_scroll_to_cursor via _apply_scroll) are marked with
_internal_top and skipped, so key-nav/click don't double-move the cursor. All
scroll sources funnel through the one watch point.

Pilot `hex` scenario: put the cursor mid-viewport, scroll 30 rows, assert its
screen row is unchanged (+ clicks still land on the exact byte). 10/10 pass.

0 files changed, 0 insertions, 0 deletions