feat: add MCP corruption corpus coverage artifact

Add a deterministic MCP corruption corpus coverage artifact.

- generate artifact-backed coverage from committed manifest, fixtures, contracts, and ContractValidator
- prove 18/18 materialized MCP corruptions are native-covered
- record 0 adapter gaps across 6 operators, 3 fixture families, and 6 failure labels
- add reproducibility and regression tests for the committed artifact
- harden generator input parsing and keep tests from rewriting committed artifacts

No runtime orchestration, LLM judge, embeddings, fuzzy matching, dashboard, showcase, dependency, or taxonomy changes.

Author: ProfRandom92