Add an "Enum evolution" section to schema-language.md covering the four
supported shapes and their tiers, plus the unsupported cases (non-String
scalar change, interface enums, in-place variant rename). Record the new
ChangeEnumConstraint migration step. Add OG-MF-105 / OG-MF-107 to the
schema-lint code table and clarify OG-MF-106 as a genuine scalar change.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>