Add Log.child(), Log.problem(), and injection-point-aware DI factory

- Injector.bindFactory(Class, Function<Key, T>): factory-based binding
  that receives the full injection-point Key (including @Named qualifier),
  enabling qualifier-derived instances like hierarchical logger names.

- Log.child(String): returns a child logger with hierarchical naming
  (e.g. "compiler:compile" → "compiler:compile.diagnostics"), useful
  when plugins delegate to sub-components that need independently
  filterable log output.

- Log.problem(BuilderProblem): reports a structured problem to the
  diagnostic collector with dedup key, suggestion, and documentation
  URL, while also logging at the appropriate level. Uses a thread-local
  flag to prevent double-counting by BuildReportCollector's WARN
  auto-promotion.

- DefaultMavenPluginManager: switched from bindInstance to bindFactory
  for Log injection, so @Inject @Named("diagnostics") Log in a
  DI-managed plugin component gets "compiler:compile.diagnostics".

- Fixed pre-existing bug: DefaultLog.warn(Supplier, Throwable) was
  calling logger.info() instead of logger.warn().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>