feat: add codegen support for @BoundAny field generation

[david-waltermire]

Summary

• Add AnyInstanceTypeInfoImpl to generate @BoundAny-annotated IAnyContent fields during code generation
• Update AssemblyDefinitionTypeInfoImpl.processModel() to handle IAnyInstance from assembly definitions
• Add codegen test verifying generated class structure matches the hand-written AnyAssembly fixture
• Add Claude rule documenting the full scope of changes required when adding new Metaschema model features

This complements PR #657 which added runtime <any/> support (core model, databind I/O, schemagen) but did not update the code generator. Without this change, generated binding classes for assemblies with <any/> would not capture unmodeled content.

Closes #220

Test plan

• AnyCodegenTest verifies generated class has @BoundAny field of type IAnyContent with getter/setter
• All 236 databind tests pass
• Full CI build (mvn clean install -PCI -Prelease) passes with 0 SpotBugs/Checkstyle violations

Summary by CodeRabbit

• New Features

  • Code generation now supports assemblies with unmodeled (flexible) content, producing appropriately-typed fields with the needed annotations.

• Tests

  • Added tests verifying generation of annotated fields and corresponding accessors for flexible-content assemblies.

• Documentation

  • Added a comprehensive Metaschema feature checklist covering mandatory/conditional areas and verification steps.