Honour region subtags

[taj-p]

LLM Contributions: Regression testing, CI changes (GPT 5.5 Sol).

Fixes #748.

Before	After
Subtag not honoured (same glyphs throughout)	Subtag honoured

Changelog

Fixed

• Honours script and region sub tags for correct shaping of, for example, zh-Hans and zh-Hant.

[taj-p]

If someone approves, please merge yourself. If you cLGTM with tests removed, I'll remove tests and then merge.

[nicoburns]

I'm personally not a fan of the "create a custom font to test" pattern. If we're going to test this, I'd much rather we find some real fonts to test with. That costs a little repo size, but it makes for a more realistic test and less code to maintain. If we wanted to keep sizes down then we could always subset those fonts.

[taj-p]

I'm personally not a fan of the "create a custom font to test" pattern. If we're going to test this, I'd much rather we find some real fonts to test with. That costs a little repo size, but it makes for a more realistic test and less code to maintain. If we wanted to keep sizes down then we could always subset those fonts.

That's fine by me - preferring realism over non-native speakers being able to easily determine correctness is a choice I am happy with. I'll update the tests accordingly.

[DJMcNab]

Sticking a 15MB font file into the repo is at the very least a surprising choice to make without discussion (e.g. at text office hours). It's not the end of the world, but personally I'm inclined to force push a revert; this is a repository size cost we would otherwise have to pay until the end of time.

This is under the assumption that the current font is not subsetted. If this is subsetted, it seems crazy to me that 15MB is the minimal size, but I don't know if there's some table which can't actually be subsetted in play here.

[petamoriken]

Since Web Platform Tests subsets the Noto Font and uses it for testing, I think it is probably possible to reduce the file size.
https://github.com/web-platform-tests/wpt/tree/0cc6a7e1904a69dae98c86412b15bc81256279f8/fonts/noto/cjk

[taj-p]

Sticking a 15MB font file into the repo is at the very least a surprising choice to make without discussion (e.g. at text office hours). It's not the end of the world, but personally I'm inclined to force push a revert; this is a repository size cost we would otherwise have to pay until the end of time.

This is under the assumption that the current font is not subsetted. If this is subsetted, it seems crazy to me that 15MB is the minimal size, but I don't know if there's some table which can't actually be subsetted in play here.

I'm happy if you want to force push a revert and I can add this PR back with a subset. Maybe worth force pushing earlier than later?

[DJMcNab]

Done, this has now been force-push reverted.