adding folder structure and READMEs

mechanical/README.md

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+# /mechanical
+
+Mechanical design including enclosures and 3D files.

mechanical/cad/README.md

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+# /mechanical/cad
+
+Editable CAD files (e.g., FreeCAD, Fusion360).

mechanical/drawings/README.md

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+# /mechanical/drawings
+
+2D mechanical drawings (e.g., DXF, PDF).

mechanical/enclosure/README.md

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+# /mechanical/enclosure
+
+STL/STEP files for 3D-printable enclosures.

mechanical/stl/README.md

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+# /mechanical/stl
+
+Exported STL files ready for printing.