Retire the @enum directive; infer enum Java backing from producers

The @enum(enumReference:) directive links an SDL enum to a Java enum class, but that backing is inferable authoritatively from the use site: an enum-typed coordinate resolves a producer (a column javaType, a @service signature type, an accessor return type) whose Java type is the backing. This is the same producer-reflection that retired @record. So an authored @enum value adds no information graphitron cannot derive and can only contradict the inferred truth, a pure misconfiguration surface. Retire it the way @record / @notGenerated / @multitableReference are retired: keep the declaration so the parser does not choke, reject any application at classify time with a migration message, and derive the enum’s backing (the EnumBacking roll-up in R333’s Enum facts) from producers instead. The remaining failure mode is genuine, not a config artifact: two use sites of one enum resolving to different backings (one-enum-one-type), which surfaces as a typed rejection. Note the backing need not be a Java enum: an SDL enum may map to a String (varchar) or numeric (integer) column, so the inference generalizes over backingType, not just jOOQ-generated enum classes.

Model decided in R333 (Enum facts); this item is the code retirement and the producer-inference path that replaces the directive. Coordinate with R261 (wire-coercion cast guard), which already owns the enum-name-divergence rejection at the column-binding site.