Correct snowy grass slab caps on 1.18.2

[SkyBlade1978]

Summary

• render snowy grass slab tops with the untinted vanilla snow texture
• preserve the existing snowy grass side edges
• bump the Forge 1.18.2 release to 1.1.1.118021
• add a resource regression test and update release documentation

Verification

• clean check, build, Javadocs, release audit, checksums, and Eclipse isolation passed
• all 15 required GameTests passed
• two clean builds produced identical main, sources, Javadocs, and checksum artifacts
• main jar SHA-256: 1B76AE9C5172B78D5DD9934713790BB6FDEEF3D95F140E1D206A881674FF59DF