refactor(mcp): integrate evaluator with production service

[Lauritz-Timm]

Refactors the MCP evaluator from the original standalone cleanroom/eval package into the workspace crate crates/icm-mcp-eval, while preserving the frozen evaluation contract.

What changed

• Ordinary modern MCP scenarios use production icm_mcp::McpService, ConnectionState, JsonRpcMessage, and icm-store directly in-process.
• Legacy byte-exact wire goldens, malformed/oversized transport frames, isolation, CLI/provider, proxy, HTTP, and topology scenarios remain isolated candidate-process tests.
• Production protocol constants, metadata validation, lifecycle error codes, and capped framing are shared rather than duplicated in the evaluator.
• Frozen contracts, fixtures, goldens, normalization rules, checksums, and the complete 294-scenario inventory remain unchanged.
• Added workspace CI/design checks and updated evaluator/MCP architecture documentation.

This supersedes the original PR description that called the evaluator standalone and product-independent: fixture construction remains evaluator-owned and independent, while the evaluator package intentionally links only production icm-mcp and icm-store for its direct service lane.

Validation

• cargo fmt --all -- --check
• cargo clippy --locked --offline --workspace --all-targets -- -D warnings
• cargo test --locked --offline --workspace --no-fail-fast (serial test threads for the existing marginal store performance assertion)
• verify-design: 294 scenarios, 31 tools, 11 structured tools
• Two-root baseline self-test: equal reports, 199 PASS / 95 UNSUPPORTED_BASELINE

The current candidate branch does not yet expose icm-cli provider; provider scenarios therefore remain explicitly unsupported in baseline mode and require a provider-capable candidate for full acceptance.