MCP upgrade 1/6: Isolated contract evaluator

[Lauritz-Timm]

Part of the six-phase full MCP service upgrade.

Why

The upgrade needs a repeatable acceptance surface that runs without network access, provider installations, user configuration, or machine-specific paths.

What

• Adds an offline evaluator with generated mock homes, stores, provider files, and proxy state.
• Pins the protocol, output, provider, proxy, and normalization contracts.
• Exercises 294 scenarios in roots containing spaces and Unicode.
• Records the corrected frozen baseline and compares candidates by normalized behavior.

Dependency

Standalone from develop; no production phase depends on merging this evaluator.

Verification

• Evaluator tests passed 44/44; formatting, strict Clippy, design verification, and diff checks passed.
• The approved frozen baseline receipt was reused without rerunning the baseline.
• The final integrated candidate passed 294/294 scenarios in each root with normalized equality.