Sync objcryst upstream and add standalone unit-test workflow

[vincefn]

Summary

• update the objcryst submodule to the latest upstream commit
• add a standalone test/ harness that builds and runs upstream ObjCryst unit tests against libobjcryst
• add a GitHub Actions workflow to build libobjcryst and run the standalone tests on pull requests
• move the upstream sync notes into the 2026.2 draft changelog entry and bump the fallback version

Notes

• the standalone harness currently skips the powder ground-truth regression cases that differ under libobjcryst's double-precision build
• generated test/bin/ and test/obj/ artifacts are gitignored

[vincefn]

@sbillinge@stevenhua0320 - this is ready for review. In addition to the upstream update, there is now a CI, which runs in double precision accuracy.

You can squash & merge if that's fine for you. After the merge, you need to tag the HEAD as v2026.2 so the build works (the CI ignores FALLBACK_VERSION).

[sbillinge]

Thanks @vincefn this is great. Wow, that was a lot of work to get that going! For pyobjcryst if you remember we moved from v2025.1 pattern for the release tag to 2025.1.0 pattern so it respected the date.version pattern you were using but also allowed all our automated release workflow machinery to work. I think this is not needed here per se because the release workflow is a bit different perhaps, but is there a conversation to be had about synchronizing that and using 2026.2.0 instead of v2026.2? I am ok either way if it doesn't break our automated workflow downstream.

[vincefn]

2026.2.0 is fine for me as well - I guess libobjcryst also needs an AGENTS.md file so I don't forget.

If you change this also change the fallback version.

[vincefn]

I updated the version to 2026.2.0

[sbillinge]

This looks good to me.