Add M2 Screw Tolerance Test and update docs

[jamro]

Context

M2 screws currently thread directly into the printed PLA/PETG parts. The model uses the screw_thread_diameter parameter to control the pilot-hole diameter.

Print tolerances vary between printers, materials, and slicer settings. On some printers the resulting holes may be large enough that M2 screws slide through instead of cutting a thread.

Changes

• Add a small, quick-to-print M2 screw tolerance test piece.
• Include several holes with slightly different diameters so users can determine the best fit before printing the full model.
• Clearly mark each hole with its diameter.
• Use the test result to configure screw_thread_diameter.
• Add/document a screw_thread_diameter parameter for parts

Documentation

Update the assembly/printing documentation to explain:

• M2 screws normally thread directly into the printed plastic.
• Printer tolerances can require adjusting screw_thread_diameter.
• How to use the tolerance test before printing the full model.
• Recommended tested values for direct M2 threading.

Acceptance criteria

• Printable M2 tolerance test added
• Direct-thread hole sizes tested
• screw_thread_diameter documented
• Printing/assembly instructions updated