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.

docs/ux/mockups/gen_offer_table.py

@@ -7,7 +7,7 @@ The map exists ONCE. The mockups and the shipping menu read the same rows in the
 order from the same file, so a drawing and the product cannot drift apart — which is the
 only way row constancy (charter 4.1) survives contact with a codebase.
 
-Output: src/slic3r/GUI/DesignOffer.hpp, checked in and never hand-edited.
+Output: src/slic3r/GUI/CAD/DesignOffer.hpp, checked in and never hand-edited.
 """
 
 import json
@@ -17,7 +17,7 @@ HERE = os.path.dirname(os.path.abspath(__file__))
 UX = os.path.dirname(HERE)
 REPO = os.path.dirname(os.path.dirname(UX))
 ATLAS = os.path.join(UX, "tool_atlas.json")
-OUT = os.path.join(REPO, "src", "slic3r", "GUI", "DesignOffer.hpp")
+OUT = os.path.join(REPO, "src", "slic3r", "GUI", "CAD", "DesignOffer.hpp")
 
 # selection id -> C++ enumerator
 ENUM = {