Add new backfaceColor argument to appearance stdlib fn

[adamchalmers]

Previously, if you set a surface's appearance, it would set the appearance of both the front face and backface. But users might want to visually distinguish front and backfaces.

So now they can use

appearance(color = "#ff0000", backfaceColor = "#112233")

Includes the Typescript codebase support so you can edit this in the modals

[vercel]

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Project	Deployment	Actions	Updated
modeling-app	Ready	Preview	Oct 20, 2026 6:06pm UTC

[codspeed]

Merging this PR will not alter performance

⚠️ 2 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• no_engine_mock_execute_mike_stress_test[1000]
• no_engine_mock_execute_mike_stress_test[3000]

✅ 157 untouched benchmarks
⏩ 131 skipped benchmarks¹

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_{Comparing achalmers/customized-backfaces (04a29cd) with main (d3da241)²}

Footnotes

1. 131 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 (252827e) during the generation of this report, so d3da241 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[pierremtb]

Note for me: need to figure out why we need to override the types for string here as it should be automatic. This was the case for color already so it's not part of this PR.