test: score PIP cam bolt and clip as MCP tutor goldens

[jeffglousher]

Summary

• Headless MCP tutor goldens for #16 wrap the existing print-in-place cam bolt and drawer clip (demo_export_pip_3mf), not a cube.
• CI-scorable assertions: 4 / 3 named bodies, 0.4 mm AABB clearance, ZIP/PK 3MF, and slicer-variant Metadata (Bambu / Orca / Prusa / Cura / standard).
• Short human-facing quest copy in docs/mcp-harness.md and docs/agent-mcp.md. No badges or streaks.
• UI tutor narration of the same steps remains on [P1][education] Tutor quests = golden MCP scenarios (gamified without cringe) #16 (this PR is the MCP/CI slice only).

Refs #16

Test plan

• cargo test --manifest-path mcp-server/Cargo.toml tutor_quest_pip (Linux OCCT) — all 3 passed
• solid_export_3mf_returns_base64_payload and set_body_appearance_from_preset_then_exports_3mf still pass
• Windows CI mcp-server.yml (cargo test --manifest-path mcp-server/Cargo.toml)

Notes

• Headless: no cad_attach. Demo tool does not mutate the document.
• Cube / box-hole quests were dropped; these are the MCP-adjacent models that look like parts.

[jackControls]

Reviewed the three tutor golden tests and their assertions around body counts, 0.4 mm clearances, ZIP/3MF contents, slicer metadata variants, and non-mutation of the working document. All three focused tests pass locally. Approved; I did not merge.