Add experimental faceOf() stdlib function

[jtran]

Extracted from #9557.

Motivation

Problem 1. Right now, when you sketch a profile and tag an edge, then extrude, the tag is ambiguous. Does it refer to the edge or the wall face that it becomes? Our docs say that extruding morphs one into the other 🤦 So what happens to the edge? You can't refer to it anymore? This specifically came up in the design for GD&T where users may want to annotate arbitrary geometry, so we can't infer that they mean one or the other.

Problem 2. If you want to use the face for anything significant like sketching on it, the tag is lacking information. Ever notice how you must do startSketchOn(mySolid, face = myTag)? Why isn't it just startSketchOn(myTag)? Shouldn't that be enough to know what face you mean?

Proposal: A new function that bundles up all the relevant information of a face on a solid. The signature looks like this: fn faceOf(@solid: Solid, face: TaggedFace): Face. Basically, I'm trying to prevent new sketch blocks from inheriting the wonkiness of sketch v1. And for that, I'd like to be able to say sketch(on = something), where something is a face, not sketch(on = mySolid, face = myTag). But I also see this as a general way to disambiguate a face from an edge because we could make myTag default to the edge. OTOH, if you want the face, then you can say faceOf(mySolid, face = myTag).

In the future, we can make tags actually know about the solid they came from, and we could extend the function with an overload to allow faceOf(myTag). It disambiguates from the edge. No breaking change.

Implementation

The implementation already exists as a specific case of the legacy startSketchOn(). Ever notice how that function returns a Plane or Face? Weird, considering the name of the function. But it's exactly what we need for a sketch surface.

The parameter naming is unfortunate. faceOf(..., face = ...) We try to avoid the duplication. But we also try to follow the convention that a parameter named tag is only for creating a tag. This mirrors the signature of the legacy startSketchOn(solid1, face = tag1).

[vercel]

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Project	Deployment	Review	Updated (UTC)
modeling-app	Ready	Preview, Comment	Jan 12, 2026 8:25pm

[codspeed-hq]

Merging this PR will not alter performance

✅ 152 untouched benchmarks
⏩ 92 skipped benchmarks¹

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_{Comparing jtran/stdlib-face-of (b91df23) with main (53a168c)²}

Footnotes

1. 92 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (efbc779) during the generation of this report, so 53a168c was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩