Add helix entity references

[Irev-Dev]

We noticed that selecting a standalone helix through the face API returned an empty entity-reference payload, so the app had no stable way to identify or reuse the selected object in point-and-click commands.

This adds a narrow Helix { helix_id } entity-reference variant carrying the helix object's existing UUID. We originally modeled this as a generic 3D curve, but that exposed a broader protocol concept than we currently support; keeping the schema helix-specific fixes the concrete gap without making assumptions about future 3D curve entities.

Engine PR: https://github.com/KittyCAD/engine/pull/4882

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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  Coverage        ?   35.73%           
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  Files           ?       37           
  Lines           ?     1858           
  Branches        ?        0           
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  Hits            ?      664           
  Misses          ?     1194           
  Partials        ?        0           

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unittests	35.73% <ø> (?)

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