Parent issue: #19604

Description

Add Checkstyle checks for all rules under §3.4 - Class structure of the OpenJDK Java Style Guidelines v6.

Rules covered

From §3.4 - Class structure:

1. The recommended order of class members is: Fields → Constructors → Factory methods → Other Methods. - DeclarationOrder, ConstructorsDeclarationGrouping, OverloadMethodsDeclarationOrder
2. Related fields should be grouped together. Ordering fields primarily according to access modifiers or identifier is not required. The same applies to methods. - DeclarationOrder
3. Nested types are put at the top of the class or right before it is first used. - No checks available.

Checks to add to openjdk_checks.xml

TreeWalker-level:

<module name="TreeWalker">
  <!-- Class member order: Fields → Constructors → Methods -->
  <module name="DeclarationOrder">
    <!-- Allow ordering by logical grouping rather than access modifiers -->
    <property name="ignoreModifiers" value="true"/>
  </module>

Coverage Gaps

• Factory methods before other methods: No existing check can identify "factory methods" and enforce they come before other methods. DeclarationOrder treats all methods as one group.
• Related fields grouped together (semantic grouping): Partial coverage: ignoreModifiers="true" allows non-modifier-based ordering, but no check enforces that related fields must actually be grouped together (requires semantic understanding).
• Nested types at the top or before first use: No coverage. InnerTypeLast enforces the opposite (nested types at the bottom). No check supports "at the top" or "before first use."

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