Design tab: move the CAD sources into their own folder

Review request on PR #15238: "Place CAD-related files (e.g. CadDocument/
GeometryEngine) into a separate folder."

  src/libslic3r/CAD/     the kernel — CadDocument, GeometryEngine, the four
                         Sketch* units, SketchSolver, ThreadStandards
  src/slic3r/GUI/CAD/    the tab — DesignPanel, DesignCanvas, DesignSketchTool,
                         SketchInlineEditor, McpControl, generated DesignOffer

Pure relocation: no line of logic changes. Two include rewrites follow from it —
files that moved re-spell their own neighbours against src/ (already on the
include path), and files that did not move pick up the new folder. docs and
docs/ux/mockups/gen_offer_table.py follow the same paths.

Verified: libslic3r, libslic3r_gui and libslic3r_tests all build, CAD suite green
at 2518 assertions in 194 test cases, and the sibling fork builds identically —
17 shared sources still byte-identical, 8 diverging by their expected counts.

src/slic3r/GUI/CAD/McpControl.hpp

@@ -0,0 +1,24 @@
+#ifndef slic3r_GUI_McpControl_hpp_
+#define slic3r_GUI_McpControl_hpp_
+
+// MCP control surface (slice 1): a local JSON-RPC 2.0 server, line-delimited over a
+// Unix domain socket, that lets an external MCP bridge drive and perceive the Design
+// tab. Off unless the env var SNAPORCA_MCP is set:
+//   SNAPORCA_MCP=1                -> socket at /tmp/snaporca-mcp.sock
+//   SNAPORCA_MCP=/path/to.sock    -> socket at that path
+// All CAD work is marshalled onto the wx main thread and runs through the SAME
+// CadDocument kernel the GUI uses (no parallel engine). Slice-1 methods:
+//   describe_tools, describe_scene, extrude.
+//
+// ponytail: Unix-socket only (POSIX). Windows compiles this to a no-op; add a named
+// pipe transport when a Windows agent actually needs it.
+
+namespace Slic3r { namespace GUI {
+
+// Start the server thread iff SNAPORCA_MCP is set. Safe to call once after the
+// MainFrame + DesignPanel exist. No-op when the env var is unset or on Windows.
+void start_mcp_control_if_enabled();
+
+}} // namespace Slic3r::GUI
+
+#endif // slic3r_GUI_McpControl_hpp_