Add lightweight Gungraun CI benchmark harness

[lucarlig]

Summary

• add a dedicated Gungraun 0.19.4 workspace crate for deterministic in-process request paths
• gate pull requests on Callgrind instruction-count regressions above 5% against a commit-addressed main baseline, with an exact-base fallback
• add Linux developer tooling and document the distinction from cf-integration load tests

Benchmark cases

• successful modern server/discover
• excessive MCP standard-header rejection
• unknown tools/call route rejection
• final annotated parameter-header mismatch rejection
• valid annotated tools/call through routing, header construction, and upstream client setup
• published resources/read through routing, URI mapping, header construction, and upstream client setup

The final two cases use an unsupported backend URL scheme and fail deterministically before DNS or socket I/O.