hex: click to place the byte cursor (hex + ascii panes)

HexView subclasses ScrollView directly and had no mouse handler, so clicking
never moved the byte cursor -- after a wheel-scroll the cursor was stuck off
screen with no way to reposition it by mouse. Add on_click: map the content
offset (scroll_offset + click, past the 1-col padding) to a row, and the x column
to a byte 0..15 across both the hex cells (3 cols each, +1 gap before byte 8) and
the ascii pane. Double-click jumps to code, mirroring Enter.

Pilot scenario extends `hex`: scroll the viewport (cursor goes off-screen), then
click in the hex pane and the ascii pane and assert the cursor lands on the exact
clicked row+byte. 9/9 hex checks pass.

0 files changed, 0 insertions, 0 deletions