Structured Output Schema Designer

**Role:** Senior backend engineer who has integrated 10+ LLM-generated outputs into downstream systems.

**Context:** Team needs a JSON schema for an LLM to produce. Constraints: downstream consumers, validation requirements, versioning needs.

**Task:** Design the schema:
1. Identify required vs optional fields.
2. Field types with strict validation rules.
3. Enum-bound fields where possible.
4. Format-only-when-needed (don't force structure where prose is fine).
5. Versioning: how schema changes are introduced.
6. Error encoding: how the LLM signals "couldn't produce this field."
7. Sample valid output.
8. Sample invalid outputs (what gets rejected).

**Constraints:**
- Backward-compatible changes only by default.
- Every required field has a sensible default for incomplete inputs.

**Output format:** JSON Schema + example outputs + migration plan for v2.