The Design tab's MCP socket must not wait for someone to click the tab

Building the Design tab on first use (1750c52211) leaves m_design_panel null
until a human selects the tab. McpControl::handle_on_main refused every verb
while it was null, so start_mcp_control_if_enabled() opened the socket and
then answered "Design panel not ready" to everything — for the whole session
if nobody clicked. That is precisely the headless case the socket exists for:
the click-test rig drives this app over it with no window manager and no user.

MainFrame::ensure_design_panel() now builds the panel on demand and returns
it; the tab activation and the MCP dispatcher both go through it, so there is
one construction site rather than two. Safe to build wx controls there: that
handler is dispatched on the main thread by CallAfter, as its own comment
says.

The startup saving is untouched — a launch that never opens the tab and never
speaks MCP still builds nothing.

Verified: libslic3r_gui builds clean 745/745; fork parity green with snaporca,
which carries the same fix (McpControl.cpp is a byte-identical shared source).

src/slic3r/GUI/MainFrame.hpp

@@ -391,6 +391,11 @@ public:
     // selected, so everything the Design panel builds stays off the startup path.
     wxPanel*              m_design_page { nullptr };
     DesignPanel*          m_design_panel { nullptr };
+    // Builds the Design panel if it does not exist yet and returns it (null only before the
+    // placeholder page itself exists). Main thread only -- it creates wx controls. Both the
+    // tab activation and the MCP socket go through this: the socket is driven headlessly,
+    // with nobody to click the tab, and without this every verb would answer "not ready".
+    DesignPanel*          ensure_design_panel();
 #endif
     //BBS: GUI refactor
     MonitorPanel*         m_monitor{ nullptr };