Generate unions as private-ctor + per-variant accessors + visitor (Java-8 safe)

[OmarAlJarrah]

Problem

Kotlin sealed/permits doesn't fit Java-8 generated output, and unions need forward-compat.

Proposed change

Emit unions as a private-constructor class with one nullable field per variant, an accept(Visitor<T>) that dispatches on the first non-null arm else visitor.unknown(raw) (default unknown throws), and a retained raw node on every parsed variant. Emit the deserializer against the Serde SPI, not Jackson.

Prior art: openai-java's union types (e.g. ChatCompletionContentPart.kt) — Apache-2.0.

Acceptance

• Union codegen template + visitor
• Unknown variant preserved, not lost
• Test

Dependencies

• Blocked by Design a dependency-free four-state JSON field model for generated types #50 — retains a raw node per variant

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Priority: high · Effort: medium