Add magnetic dock lid + SO-101 cart trailer

[BenCaunt]

Summary

• Adds two N52 12×5 mm disc magnets to the rear of the flat-disk-robot lid (one on each side of the rear service opening). Both bores are radial, so the pole faces sit flush with the curved outer wall; the supporting bosses extend inward only and keep the lid inside the 216 mm envelope.
• Adds a new so101-cart project bundle: a small two-wheeled trailer whose front cradle wraps the rear arc of the disk robot with a 0.5 mm dock gap and holds matching radial magnets, so the disk robot can push or pull the cart wheelbarrow-style.
• Imports TheRobotStudio's SO-101 follower assembly STEP and places it on the cart deck for visual reference.
• Per the request, only the lid was modified on the existing disk robot — the chassis is unchanged.

New build targets

• so101-cart-deck (printable) — cradle + deck + cheek legs + magnet pockets
• so101-cart-wheel (printable) — passive 70 mm idler wheel with M5 bore
• so101-cart — cart-only reference assembly (deck, wheels, axle, magnets, SO-101)
• flat-disk-robot-with-cart — docked reference assembly

Notable geometry

• Lid magnet bores: 12.4 mm × 5.2 mm pockets at X=±50 mm, mid-wall height (Z≈34.75 mm), seated in 18 mm OD × 7 mm bosses inset 0.5 mm so the lid bbox stays at exactly 216 × 216 mm.
• Cart cradle: inner R=108.5, outer R=116.5, ±40° around the rear, height matches the lid wall + roof.
• Cart deck top is coplanar with the disk-robot lid roof so the SO-101 base sits at a known Z.

Test plan

• pytest projects tests — full suite (55 tests, including 14 new ones for the lid magnets and cart geometry) passes.
• Smoke build of so101-cart-deck, so101-cart-wheel, and flat-disk-robot-lid exports clean STEP / STL / GLB.
• Visually confirm the docked assembly in the web viewer.
• Verify in the slicer that the lid magnet bosses don't introduce overhangs that need supports.

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Generated by Claude Code

[github-actions]

SynthCAD CAD review

Compared 50cc8c6e6505 to 221fd1200a91 for flat-disk-robot.

• Interactive diff viewer
• Workflow run and generated artifacts