fix(input,examples): a linked figure's wheel anchor uses the group's live edge, and every demo verdict reaches its exit status

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.

adapters/glfw/glfw_host.cpp

@@ -12,6 +12,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <limits>
 
 namespace plot::glfw {
 namespace {
@@ -422,15 +423,22 @@ input::Controller& Host::controller_at(f32 x, f32 y) noexcept {
   const FigureLayout& layout = figure_->layout();
   const std::vector<Rect>& rows = layout.rows;
   if (rows.empty() || rows.size() != row_controllers_.size()) return controller_;
+  // The row the pixel is in, or -- when it is in none of them, which is the row gap a
+  // host has asked for with `Figure::set_row_gap()` or the figure's own margins -- the
+  // row whose rect is nearest. Selection by distance rather than by "the first that
+  // matches" is what makes a pixel in a gap belong to the row it is *near*: the rows
+  // tile the figure at the default gap of 0, so this is unreachable until a host asks
+  // for one, and a gesture in a visible gap must not be routed to row 0 by default.
   u32 row = 0u;
-  if (y >= rows.back().y + rows.back().h) {
-    row = static_cast<u32>(rows.size() - 1u);  // below every row: the nearest one is the last
-  } else {
-    for (u32 i = 0; i < rows.size(); ++i) {
-      if (y >= rows[i].y && y < rows[i].y + rows[i].h) {
-        row = i;
-        break;
-      }
+  f32 best = std::numeric_limits<f32>::infinity();
+  for (u32 i = 0; i < rows.size(); ++i) {
+    const f32 top = rows[i].y;
+    const f32 bottom = rows[i].y + rows[i].h;
+    const f32 distance = y < top ? top - y : (y >= bottom ? y - bottom : 0.0f);
+    if (distance < best) {
+      best = distance;
+      row = i;
+      if (distance == 0.0f) break;  // inside this row: nothing can be nearer
     }
   }
   (void)x;  // the rows tile the figure vertically, so y alone picks one