feat(examples): a three-row synced Figure demo, on a GLFW host that draws one

The multi-plot example the phase exists for: three plots stacked in one
Figure, sharing one x link with following armed, each row carrying its own
named signals and its own legend, driven by six StreamSeries at 500 kS/s.
Two cursors and a region with an exact statistics readout reach the direct
manipulation of spec §5 in the same frame, and PLOT_DEMO_SYNC=1 is a
self-checking mode that exits non-zero unless all three rows hold one range
and the shared right edge keeps tracking the newest sample through a zoom, a
pan on another row and more arriving data.

The GLFW host gains the figure mode the demo needs: set_figure() makes the
run loop draw the Figure instead of its own plot and route each pointer,
wheel and key event to the row it lands in, through one Controller per row
(row_controller()). All of it is opt-in; the single-plot path is untouched.

examples/CMakeLists.txt

@@ -9,6 +9,16 @@ if(TARGET plot::glfw)
   plot_apply_warnings(plot_glfw_demo)
 endif()
 
+if(TARGET plot::glfw)
+  # The multi-plot example: a three-row `Figure` sharing one x axis. It needs the GLFW
+  # host's figure mode (the host draws the figure and routes each row's gestures), so it
+  # is guarded by the same target as the single-plot demo above; its producer thread is
+  # the only extra dependency.
+  add_executable(plot_multiplot_demo multiplot_demo.cpp)
+  target_link_libraries(plot_multiplot_demo PRIVATE plot::glfw Threads::Threads)
+  plot_apply_warnings(plot_multiplot_demo)
+endif()
+
 if(TARGET plot::imgui)
   # The demo owns the window, the ImGui context and the frame loop itself, so it
   # links the toolkit and its backends directly; the adapter is only the widget.