Update ObjCryst for 2026.3.0

[vincefn]

Summary

• update the ObjCryst submodule from 90a3afc to 30fc8df
• add the 2026.3.0 changelog with upstream PR attribution
• bump the fallback version to 2026.3.0.post0

Notable upstream changes

• add fixed-histogram powder backgrounds and the Thompson-Cox-Hastings pseudo-Voigt profile
• add Rwp convergence controls, Rw history, and refinable parameter proxies
• fix anisotropic pseudo-Voigt construction and MonteCarloObj autosave filename handling

Validation

• built the updated library locally
• ran the standalone upstream unit-test harness successfully

[vincefn]

@clemisch @sbillinge let me know if that's ok for you, this would lead to a ne release of pyobjcryst

[clemisch]

Thanks, sounds good to me! There's nothing left I want to upstream for now. (I haven't used the new TCH profile a lot but it does work well so far)