docs(m8): freeze EvalCase v1 contract for contributor eval work

[AllenXiao95]

Summary

Freeze the maintainer-side prerequisite contract shared by #13 and #15 before contributor implementation hardens around incompatible interpretations.

This PR is intentionally contract-only. It does not implement fixtures, deterministic evaluators, JSON Schema validation, provider/model calls, baselines, or AgentRuntime integration.

Contract decisions

• EvalCase schema_version = 1
• explicit dev/ vs locked/ case boundary
• stable case-ID convention
• required/optional field semantics
• explicit provenance/license requirements
• assertion objects instead of bare strings
• [M8][contributor] Add synthetic placeholder/protocol eval fixtures and deterministic checks #13 required assertions:
  • protocol_complete
  • placeholder_integrity
• placeholder_integrity checks identity and multiplicity, so %s %s -> %s fails; it replaces the weaker proposed placeholder_set wording
• reserved deterministic assertion names for later [M8] Add reproducible i18n Prompt evaluation harness and baseline reports #9 work without requiring [M8][contributor] Add synthetic placeholder/protocol eval fixtures and deterministic checks #13 to implement them
• case definition is separated from candidate output
• deterministic AssertionResult evidence contract is frozen separately from case input

Contributor boundaries

• [M8][contributor] Add synthetic placeholder/protocol eval fixtures and deterministic checks #13 owns synthetic dev fixtures plus provider-independent protocol/placeholder checks.
• [M8][contributor] Add JSON-schema validation for public i18n eval cases #15 owns executable offline schema validation against this same v1 meaning.
• [M8] Add reproducible i18n Prompt evaluation harness and baseline reports #9 owns later evaluator/report/baseline runtime and additive assertion vocabulary.

Docs

Adds matching Chinese/English contract documents and links both from the MkDocs design navigation.

Closes #32
Refs #13
Refs #15
Refs #9
Refs #30