Decouple codegen layer from builder internals in processor (phase 0) #347

Subtask of #233 — the SPI seam inside processor, no file moves.

Implemented by PR #299:

• ClassCodeGenerator SPI; RoasterCodeGenerator implements it via GenerationEnvironment (env, logger, tracker, GenerationIssueReporter) + RoasterSourceFormatterFactory instead of ProcessingContext.
• MapperOptions record replaces mapper reads of scope resolver/configuration.
• findConstructorForBuilder takes the marker annotation as a parameter.
• ProcessingLogger verbosity via named verbose(env) factory (no decision boolean).
• CompilerOption interface; readBuilderConfiguration moved to BuilderConfigurationReader.
• Phase constants out of PerformanceTracker into ProcessingPhases/GenerationPhases.

Zero behavior change; generated example builders byte-identical.