[M8][contributor] Add JSON-schema validation for public i18n eval cases

[AllenXiao95]

Parent roadmap

Contributes to #9 ([M8] Add reproducible i18n Prompt evaluation harness and baseline reports).

Goal

Provide a small schema-validation layer for public evaluation cases so malformed fixtures fail before any model or scoring code runs.

Scope

Define a versioned schema for the stable core fields already described by M8:

• schema_version
• stable id
• source/target locale
• domain
• source text
• optional context
• glossary/required tokens
• accepted references when present
• assertions
• provenance kind/license/source metadata
• difficulty/tags

Add validation tests for representative valid and invalid cases. Validation must run offline and should produce actionable field-level errors.

Non-goals

• No evaluator runtime.
• No model calls or semantic scoring.
• No baseline comparison.
• No external dataset import.

Acceptance criteria

• Public eval cases are validated against a versioned schema.
• Invalid IDs, missing provenance/license data, malformed locale fields, and unsupported assertion names fail with actionable diagnostics.
• Validation is provider-independent and network-free.
• Tests cover valid and invalid examples on Windows and Linux.
• The schema remains compatible with the contributor fixtures in [M8][contributor] Add synthetic placeholder/protocol eval fixtures and deterministic checks #13 or provides a documented migration path.

Coordination

Coordinate with #13 and #14. This task is useful only if it converges on the same schema used by the parent roadmap #9 rather than creating an alternative format.

[AllenXiao95]

Maintainer prerequisite #32 is complete and merged via PR #33 (merge a003db719fc77bfb547625c22722a3582feb18b2).

Please use the merged EvalCase v1 field semantics and assertion vocabulary as the source contract for #15. #15 should encode that contract into actionable offline validation; it should not introduce a parallel case model.

Key coordination points remain:

• schema_version is 1;
• assertions are objects rather than bare strings;
• [M8][contributor] Add synthetic placeholder/protocol eval fixtures and deterministic checks #13-required assertions are protocol_complete and placeholder_integrity;
• reserved v1 deterministic names include number_fidelity, required_tokens, forbidden_renderings, and max_chars;
• provenance/license is required for public cases;
• dev vs locked data boundaries are explicit;
• case input schema and deterministic assertion-result schema are separate concerns.

The schema-validation task is now unblocked from the maintainer-contract side.