feat(generator): DiffTraverser/DiffVisitor for paired tree comparison (C36)

[jsenko]

Summary

New infrastructure for paired tree traversal — walks two trees of the same spec version in parallel, dispatching field-level diffs to a pluggable DiffVisitor.

Base classes (visitors/diff/)

• DiffVisitor — abstract visitor with methods for each field type: diffPrimitive, diffEntity, diffUnion, diffList, diffMap. Strategy selection via getPairingStrategy(propertyName).
• AbstractDiffTraverser — shared diffMap/diffList/diffEntityField helpers with auto-recursion into matched pairs.
• CollectionDiff<K,V> — diff result with added/removed/matched entries.
• PairingStrategy<K,V> — pluggable collection pairing. Built-in: KeyPairingStrategy (maps), IndexPairingStrategy (lists).

Generated per spec version

• CreateDiffTraversersStage — generates XxxDiffTraverser per spec version (e.g., JD7DiffTraverser), following the same entity-property iteration as CreateTraversersStage.
• Each entity gets a traverseXxx(original, updated) method that dispatches to the visitor.
• traverseNode(original, updated) dispatches by instanceof to the correct entity method.

Example generated output

public void traverseFullSchema(JD7FullSchema original, JD7FullSchema updated) {
    if (!visitor.visitEntityPair(original, updated)) return;
    visitor.diffPrimitive("title", original.getTitle(), updated.getTitle());
    this.diffMap("properties", original.getProperties(), updated.getProperties());
    this.diffList("allOf", original.getAllOf(), updated.getAllOf());
    this.diffUnionField("type", original.getType(), updated.getType());
    // ... all fields
}

Context

C36 Phase 1-2 in #1042. Phase 3 (compat checker migration) will be a follow-up.

Test plan

• All 1129 data-models tests pass
• Generated code compiles (FullGeneratorTest)
• Synthetic snapshot tests pass