Extract framework-agnostic builder/DTO model into its own project (resolves Sonar duplication gate)

[AndreasIgel]

Summary

The SonarCloud new-code Duplication density on main is ~7.6% (limit 3%), failing the quality gate. The duplication is by design: the processor carries two parallel model families — a generation-side model (BuilderDefinitionDto, BuilderMethodDto, FieldDto, ...) and a rendering-side model (GenerationTargetClassDto, MethodDto, ...), bridged by BuilderToGenerationTypeMapper. They are ~90% structurally identical, which Sonar counts as duplication.

Rather than force a base-class refactor, we will resolve this by extracting the fundamental, framework-agnostic model into its own project.

Goal

Provide a standalone, reusable foundation that any annotation processor could build on:

• A clean set of DTOs describing the class/builder/field/method model that users work with.
• Hide the concrete framework tooling used to actually emit source (currently Roaster) behind that model, so consumers depend on the DTOs, not on the code-generation library.
• Easy setup: define the DTOs, hand them to the generator, get generated classes — without wiring up framework internals.

Proposed direction (to refine during design)

• New module/project (e.g. codegen-model / codegen-core) containing the neutral DTOs + a generator SPI.
• Move/merge the currently-duplicated generation & rendering models into that single model, eliminating the parallel duplication.
• simple-builders processor depends on it and maps @SimpleBuilder analysis results onto the shared model.
• Keep the Roaster-based emitter behind an interface so it can be swapped.

Acceptance criteria

• Shared model extracted to its own project/module; the parallel generation/rendering duplication is removed at the source.
• SonarCloud new-code Duplication condition no longer flags the parallel DTOs.
• simple-builders behavior and generated output unchanged (generated example builders byte-identical).
• Documented, minimal setup for defining DTOs and generating classes without exposing the framework tooling.

Notes

Until this lands, the duplication gate condition is accepted (agreed with maintainer). Related quality-gate items are tracked separately: NPE reliability fixes and the fork-sonar.yml security analysis task.