chore: uptake mereology across the project

[deer]

uptake Workday/base.build#58

Wire the mereology Composite contract (parts() / composition()) onto descriptors, type usages, expressions, and statements throughout the codemodel. This makes the entire AST uniformly navigable — callers can transitively walk any subtree by type without writing custom visitors.

Scope

• codemodel-foundation: TypeDescriptor, all TypeUsage subclasses, AbstractTraitable, CallableDescriptor, ThrowableDescriptor
• hierarchical-codemodel, objectoriented-codemodel: ParentTypeDescriptor, FieldDescriptor, MethodDescriptor, ConstructorDescriptor, ParameterizedTypeDescriptor
• expression-codemodel: binary/unary arithmetic and logical, comparison, Cast, FunctionUsage
• imperative-codemodel: Assignment, Block, If, Return, While
• jdk-codemodel: all statement nodes (Assert, CatchClause, DoWhile, EnhancedFor, For, LocalVariableDeclaration, Switch, Synchronized, Throw, Try), MethodBodyDescriptor

Tests: MereologyTests in expression-codemodel, imperative-codemodel, and jdk-codemodel; integration tests in JdkInitializerTests confirming end-to-end traversal over a live JDKCodeModel, including var-type
inference and NewObject discovery via PersonFactory.