Add MCP control surface (slice 1): socket server + describe/extrude

Predispose the Design tab to run under an external MCP agent. New
McpControl.{cpp,hpp} embeds a line-delimited JSON-RPC 2.0 server over a
Unix domain socket, off unless env SNAPORCA_MCP is set. Requests are
marshalled onto the wx main thread and run through the SAME CadDocument
kernel the GUI uses, via two thin DesignPanel hooks (mcp_doc /
mcp_after_change) — no parallel engine.

Slice-1 methods: describe_tools (introspection -> the bridge builds tool
schemas), describe_scene (feature tree + per-body bounding boxes), and
extrude (centred rectangle sketch -> new solid). Every op returns its
full post-state. POSIX-only; Windows compiles to a no-op.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/slic3r/GUI/DesignPanel.hpp

@@ -37,6 +37,11 @@ public:
     explicit DesignPanel(wxWindow* parent);
     void on_tab_shown();        // re-sync bed to the active printer when the Design tab is activated
 
+    // MCP control hooks: let the external control server (McpControl.cpp) drive and
+    // perceive the SAME kernel the GUI uses. Called only on the wx main thread.
+    CadDocument& mcp_doc()        { return m_doc; }            // live document (read + mutate)
+    void         mcp_after_change() { after_tree_edit(true); } // refresh tree + viewport + status
+
 private:
     enum class Tool { None, Sketch, Extrude, Dressup, Hole, Thread, Shell, Revolve, Sweep, Pattern, Plane, Loft, Draft, Boolean, Cut, Insert };
     // Plane tool: which datum reference the next solid pick fills (declared early so the