Add a versioned invariant summary contract

[LittlePeter52012]

Summary

• add a Draft 2020-12 JSON Schema for the existing results/invariant_summary.json artifact
• add a mathematically checked known-answer fixture for the boundary of a 2-simplex
• require classification caveats or theorem evidence, non-null values for nonfailed results, and repair evidence for unsettled results
• document the schema, fixture, and development dependency setup in both package READMEs
• fix the English and Chinese README links to the repository-root LICENSE
• add regression tests for local links, schema validity, semantic result boundaries, and forward-compatible extensions

Compatibility and scope

• this pull request introduces the initial version 1 schema; it has not previously been published on the base branch
• additional properties remain allowed so downstream tools can extend records without changing the public core
• the schema contains result evidence only; backend selection and workflow implementation remain outside the interchange format
• jsonschema remains a development-only dependency; the Skill keeps zero runtime dependencies

Testing

• python3 -m unittest discover -s tests -v (18 tests passed)
• conda run -n math python3 -m unittest discover -s tests -v (18 tests passed)
• package validation passed
• JSON parsing and git diff checks passed