Review fix pass 1 on the task 7 work; four findings, all closed.
1. The Qt demo's region sat across the middle of the window, which is exactly
where its own scripted input check pans: after the check's box zoom the press
opened DragMode::MoveRegion, the x range never moved, and the check printed
'input check pan FAIL' -- under an exit 0, because the verdict was printed and
not returned. The region now rides the left sixth (as the GLFW demo's does),
both checks pass, and the verdict is wired to the exit status. The same audit
found two more printed-only verdicts -- the GTK demo's scripted-resize layout
check and its screenshot write (and Qt's) -- now wired too. What stays
printed-only is the screenshot line of the GLFW host and the ImGui widget,
where the dump happens inside the adapter's frame loop.
2. A follow-ing link group was invisible to the input layer: a member's own axis
must be XFollow::Off (link_x() refuses a Live member), so Controller's
x_zoom_anchor() extrapolated a coordinate from the pointer instead of using
the live edge and a wheel over a row's axis strip walked the window back by
19% of its width (behind 0.000000 -> 0.285809 s). The group now publishes its
own edge to every member (Axis::live_owner + Plot::set_x_live_owner, written
by Figure::push_link, cleared when the group stops following or the row
unlinks), and x_zoom_anchor() reads it. Public-header addition, called out.
Measured after: behind 0.012992 s and the window's right edge bit-identical to
the edge the group published at the gesture.
3. The multiplot summary printed x_link_count() -- a number of groups -- under
'rows in it'. It now reports the members and the group count separately, and
every row line carries its own group id.
4. Host::controller_at() promised to clamp a pixel in a row gap to the nearest
row and only clamped below the last one, so a gap pixel went to row 0. It now
selects by distance, and tests/test_glfw_host.cpp pins it with a 40 px gap --
the first coverage of the host's figure routing, which needs no window.
New cases: tests/test_controller.cpp x2 (the anchor and the hint's lifecycle),
tests/test_glfw_host.cpp x3 (routing, the gap, the single-plot fallback): 229
cases, 0 failed, ASan clean. All five demos exit 0, the three self-checking
modes exit 0, and the amended assertions were confirmed failing against the
pre-fix behaviour.