Switch Rue codegen selection to explicit dialect flag

[prozacchiwawa]

Summary

• add a new rue_codegen boolean to AcceptedDialect
• switch rue_cg to gate Rue codegen on dialect.rue_codegen instead of a sentinel stepping value
• update *standard-cl-rue1* to set rue_codegen: true and use normal stepping/version values
• update explicit AcceptedDialect test initializers to set rue_codegen: false

Testing

• cargo test tests::rue::