docs: add measured M2 screw list and shopping list (sg90)

[mjspe]

The docs said exact screw lengths/counts "aren't fully documented yet" in three places. This PR documents them.

What I did: ran a geometric census over every shipped parts/sg90/3mf/*.3mf mesh, detecting cylindrical features matching the M2 pilot spec (~Ø2.1 mm). The detector was validated against the labeled holes in ScrewSizingTest.3mf (recovered 8 of 9 labeled diameters exactly).

Findings added to 3d_models/README.md:

• Per-part pilot table: 27 screw locations across 11 parts (depths 2.5–12.8 mm)
• The 5 deep chest channels (14–23 mm) are servo-wire routing, not screws
• Only the head ships servo-tab pilots; the other 4 servos are pocket-fit + horn-screw captured
• No counterbores/countersinks anywhere → pan/button-head M2s
• Shopping list: ~30× M2×6 + ~5× M2×12 (one M2 assortment kit covers it)
• Servo horn screws come in the servo bags (5 total, one per joint)

Also updated the two stale "not documented yet" pointers in docs/getting-started.md and docs/hardware-for-software-engineers/06-3d-printing-and-mechanical-build.md to link the new screw list.

Counts were measured from exported meshes rather than the Fusion source, and the README says so — treat as a verified starting point, test-fit as you go.

Summary by CodeRabbit

• Documentation
  • Added measured screw specifications and pilot-hole guidance for SG90-based builds.
  • Documented an approximate per-robot shopping list: 30 M2×6 and 5 M2×12 pan- or button-head screws.
  • Linked assembly and screw guidance across the 3D models, getting-started, and mechanical build documentation.
  • Clarified that a complete step-by-step assembly guide is not yet available.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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⚙️ Run configuration

Configuration used: Repository UI

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Plan: Advanced

Run ID: 72da4963-efb0-486c-951b-b94055b91052

📥 Commits

Reviewing files that changed from the base of the PR and between c45b281 and ae2c24c.

📒 Files selected for processing (3)

• 3d_models/README.md
• docs/getting-started.md
• docs/hardware-for-software-engineers/06-3d-printing-and-mechanical-build.md

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📝 Walkthrough

Walkthrough

The documentation now defines measured SG90 screw specifications, pilot details, and approximate shopping quantities. The getting-started and mechanical-build guides link to this information.

Changes

Screw documentation

Layer / File(s)	Summary
Screw reference and measurements 3d_models/README.md	Adds M2 screw specifications, SG90 pilot measurements, servo retention notes, and an approximate shopping list.
Build guide references docs/getting-started.md, docs/hardware-for-software-engineers/06-3d-printing-and-mechanical-build.md	Updates both guides with the approximate M2×6 and M2×12 quantities and links to the README screw section.

Priority: ⚪ Not assessed

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Suggested reviewers: jamro

Merge Risk: ⚪ Minimal · up to ae2c2

The updated screw guidance and cross-document links are consistent, with no identified impact on builds or runtime behavior.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding measured M2 screw and shopping-list documentation for the SG90 build.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[jamro]

This is fantastic! thank you! I definitely didn’t expect someone to run a geometric census over all the exported models to fill this gap 😄

Really appreciate the thorough work here. This is exactly the kind of contribution that makes the project better for everyone building it. Merging!