Genericize codegen seams; keep builder model processor-side (prep for #349) #369

Preparatory step for #349, before the model move lands.

Goal: simple-codegen must contain only framework-agnostic codegen — no builder vocabulary. Generation consumes only the generic base (GenerationTargetClassDto); the builder-domain layer stays processor-side.

Seam changes (interfaces to genericize first)

• TypeMappingContext.resolveGeneratedType — currently Optional<ResolvedBuilder>; needs a neutral return (Optional<TypeName> or a generic generated-type ref). Design needed for what ResolvedBuilder.buildCall(...)/setterPrefix provide to callers (builder instantiation emission).
• TypeName.resolvedBuilder field — neutral type + name.
• BuilderInstantiation — generic instantiation-strategy vocabulary (ConstructorCall/StaticFactoryCall are generic-shaped; AnchorFactoryCall is builder-specific).
• GeneratedBuilders registry — generic generated-types registry (Map<TypeName, TypeName> semantics already).

Builder model placement

Builder-domain types stay processor-side (processor.model), never move to codegen: BuilderConfiguration, BuilderDefinitionDto, BuilderMethodDto, BuilderNestedTypeDto, BuilderToGenerationTypeMapper, ResolvedBuilder, BuilderInstantiation, GeneratedBuilders.

Preference order for modelling the split: separate type + mapping > composition > inheritance (last resort). Builder DTOs may freely use codegen vocabulary (TypeName, FieldDto, MethodDto) as field types or produce them at the boundary (BuilderToGenerationTypeMapper → GenerationTargetClassDto at finalization, as today). Where a builder type needs generic-shaped state, prefer holding it (BuilderDefinitionDto already holds/produces a GenerationTargetClassDto) over inheriting it — inheritance between the two model families only if neither mapping nor composition fits. (Extending a DTO to add a property remains a good pattern; this rule targets model-family hierarchies, not the extends mechanism.)

Genuine duplication only where codegen cannot see the other side (anti-corruption seam): the codegen.enums mirror of core.enums is the pattern — correct for the future repo split (#363).

Acceptance

• simple-codegen/simple-codegen-roaster compile with zero *Builder* types and zero builder references.
• SPI boundary remains generateClass(GenerationTargetClassDto, SourceFormatter).
• Generated output byte-identical; all tests green.