Align Choice validation and prepare initial release

[JAORMX]

Summary

• Enforce the documented Choice highest-probability rule, accepting ties and preserving empty labels.
• Keep all existing Score, finite/range, membership, and key validation. Do not invent probability-sum or Score-equality tolerances.
• Check in the official OpenAPI 0.2.0 snapshot with SHA-256 provenance and offline checks.
• Prepare draft v0.1.0 release notes, module verification instructions, and an API drift checklist. This PR does not publish a release.

Official contract cross-check

Rechecked https://api.typesafe.ai/openapi.json and https://docs.typesafe.ai/api plus the official retry documentation. ChoiceAnswer explicitly requires the highest-probability option. Usage nonnegativity is semantic token-count validation, not an OpenAPI minimum. Numeric tolerances remain unspecified. Existing Score structural checks are retained.

Verification

Offline unit tests, race tests, vet, both 10-second fuzz smoke targets, and independent spec/security/DevEx/QA review passed. Tests cover ties, exact small differences, extra answers, nil responses, preserved usage, and empty labels. No live inference.

Stacked on #4, which depends on #3. Merge in that order and retarget bases as needed. No guardrails or delegated routing implementation is added to the SDK.