Update enum payload syntax to colon form

[neysofu]

Summary

• switch parser handling and legacy docs/tests to require Variant: Type payload syntax with unit variants remaining bare
• refresh stdlib references, fixtures, and formatter snapshot to the new enum form, covering JSON enums, option/result helpers, hash collections, and IO helpers
• add parser tests for the new syntax and rejection of legacy parentheses while updating AGENTS guidance

Testing

• Not run (not requested)