Provide a reusable fail-soft recursive validator skeleton for generator output

[OmarAlJarrah]

Problem

Spec/schema validators want a consistent fail-soft shape (collect errors with a path prefix rather than throwing on the first).

Proposed change

Provide a ~15-line recursive validator idiom (one-shot recursion guard + path-prefixed error list + early-return verify helper), parameterised over our own tree type. Use FQN/deterministic names for definitions to avoid simple-name collisions.

Acceptance

• Validator skeleton
• Deterministic definition names

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Priority: low · Effort: small