processor: decouple codegen layer from builder internals (phase 0)

[igel-devin-ai]

Summary

Closes #347. Part of #233.

Pure refactor, zero behavior change — decouples the code-generation layer inside processor so it no longer depends on builder-specific internals. This is Phase 0 of extracting a reusable simple-codegen library (~40 files) into its own module/artifact: do the decoupling before any git mv so the move commits stay rename-safe.

• ClassCodeGenerator SPI (new): BuilderProcessor now talks to the generator through the interface. RoasterCodeGenerator implements it and receives a GenerationEnvironment (processing environment, logger, performance tracker, GenerationIssueReporter) plus a RoasterSourceFormatterFactory instead of the whole ProcessingContext. Strict-mode reporting keeps its semantics via context::reportBasedOnStrictMode wired as the issue reporter.
• MapperOptions record (new): carries the mapper's needs (copyTypeAnnotations flag + a generated-type resolver function), replacing mapper reads of the builder scope resolver and the builder configuration. BuilderDefinitionCreator supplies them in one place.
• findConstructorForBuilder takes the constructor marker annotation as a parameter instead of hardcoding SimpleBuilderConstructor (the builder caller passes it in).
• ProcessingLogger no longer reads compiler options itself: a private Verbosity enum plus a verbose(ProcessingEnvironment) named factory replace the implicit flag — callers decide the verbosity; the -Averbose option is read once in BuilderProcessor.
• CompilerOption interface (new) decouples CompilerArgumentsReader from the builder option enum; readBuilderConfiguration moved to BuilderConfigurationReader next to the other configuration sources.
• Phase constants moved out of PerformanceTracker (now a pure protocol) into ProcessingPhases (pipeline phases + hierarchy) and GenerationPhases (codegen sub-phases); ActivePerformanceTracker takes them as constructor arguments.

Out of scope (next PRs)

• Splitting ProcessingContext into a generic AnnotationProcessingContext + builder-specific context (separate PR — it touches many files on its own).
• History-preserving git mv of the decoupled classes into simple-codegen / simple-codegen-roaster modules (Phase 1).

Test plan

• mvn test -pl processor — all 528 tests pass
• mvn -pl example clean compile — regenerated generated-example-builder/ tree is byte-identical (confirms zero behavior change)
• license:check — all new files carry the license header