[css-gaps] More vertical-writing cases

[xfq]

https://www.w3.org/TR/2026/WD-css-gaps-1-20260227/

I did not find explicit prose covering writing-mode interactions, and the spec text does not surface vertical-writing cases. It should add more spec text and WPT coverage for writing-mode: vertical-rl, and writing-mode: vertical-lr, like for *-inset-start/*-inset-end.

[kbabbitt]

Thanks for the comments. My general intent in the Gaps spec is to defer to the specification of the container layout to determine what column and row mean, taking into account writing-mode.

I've drafted some normative changes to make this more explicit: #13732
And written some WPTs: https://chromium-review.googlesource.com/c/chromium/src/+/7705401

@xfq do these changes address your concern?