方案 C（激进）第一步：引入 pipeline 目录的导入别名层

- 新增 lib/pipeline/**（parse/validate/generate/render/output）各阶段 re-export 文件
- 不改动现有实现与导入，仅提供流程化导入入口（零行为变更）
- 后续可在别名层稳定的前提下，逐步迁移物理文件（parse/validate/generate/render/output）

质量门禁：
- dart analyze：0 error / 0 warning（仅 info）
- dart test：全部通过（203/203）

lib/pipeline/generate/apis.dart

@@ -0,0 +1,5 @@
+/// Pipeline: generate -> apis
+/// Re-export Retrofit API generator for pipeline-oriented imports.
+library;
+
+export 'package:swagger_generator_flutter/generators/retrofit_api_generator.dart';

lib/pipeline/generate/models.dart

@@ -0,0 +1,5 @@
+/// Pipeline: generate -> models
+/// Re-export model code generator for pipeline-oriented imports.
+library;
+
+export 'package:swagger_generator_flutter/generators/model_code_generator.dart';

lib/pipeline/output/generation_output_service.dart

@@ -0,0 +1,6 @@
+/// Pipeline: output -> generation output service
+/// Re-export GenerationOutputService for pipeline-oriented imports.
+library;
+
+export 'package:swagger_generator_flutter/commands/services/generation_output_service.dart';
+

lib/pipeline/parse/swagger_data_parser.dart

@@ -0,0 +1,5 @@
+/// Pipeline: parse
+/// Re-export swagger data parser for pipeline-oriented imports.
+library;
+
+export 'package:swagger_generator_flutter/parsers/swagger_data_parser.dart';

lib/pipeline/parse/swagger_fetcher.dart

@@ -0,0 +1,6 @@
+/// Pipeline: parse
+/// Re-export swagger fetcher for pipeline-oriented imports.
+library;
+
+export 'package:swagger_generator_flutter/parsers/swagger_fetcher.dart';
+

lib/pipeline/render/template_renderer.dart

@@ -0,0 +1,6 @@
+/// Pipeline: render -> template renderer
+/// Re-export TemplateRenderer for pipeline-oriented imports.
+library;
+
+export 'package:swagger_generator_flutter/core/template_renderer.dart';
+

lib/pipeline/validate/enhanced_validator.dart

@@ -0,0 +1,6 @@
+/// Pipeline: validate
+/// Re-export enhanced validator (decorator over schema validator).
+library;
+
+export 'package:swagger_generator_flutter/validators/enhanced_validator.dart';
+

lib/pipeline/validate/schema_validator.dart

@@ -0,0 +1,6 @@
+/// Pipeline: validate
+/// Re-export schema validator for pipeline-oriented imports.
+library;
+
+export 'package:swagger_generator_flutter/validators/schema_validator.dart';
+