feat(layers): add clipping to TextLayer

[Pessimistress]

This extension lets any TextLayer to provide a per-object "clipping rectangle" that defines the context box that contains the text. Text overflowing this box will be hidden. If the layer has background enabled, the background rectangle will fill the clipping rectangle instead of fit around the text. Optionally, text can be automatically aligned to the visible region of the content box.

Screen.Recording.2026-03-19.at.2.15.23.PM.mov

Change List

• ClippingTextExtension
• Example
• Documentation

[coveralls]

coverage: 91.064% (+0.03%) from 91.038%
when pulling 57b8634 on x/clipping-text
into 023afee on master.

[ibgreen]

• One concern with extensions is that we currently have no idea for how to implement these in WebGPU.
• Another is that extensions make more sense when there are multiple layers that benefit from it.
• If this is mainly a concern about adding overhead when there is no clipping we do have some other ways to deal with that the ability to control it with defines and #ifdefs

[ibgreen]

Looks great, thanks for considering and making the change!

Nit: Any new test cases that could be added for the text layer?