Perf: end-to-end Kestrel (socket-level) control-plane benchmark harness

## Scope
An **end-to-end, socket-level** control-plane benchmark harness (over Kestrel), complementing the current handler-level benchmarks which deliberately measure the handler in isolation (no Kestrel, no sockets).

## Status (2026-07-21)
Not started, actionable. Adds a real-socket end-to-end measurement over Kestrel alongside the existing in-process handler-boundary benchmarks.

## Sources
`docs/arazzo/guides/execution-host.md`; `docs/BenchmarkGuide.md`.

Part of #803.

Start with docs/arazzo/guides/execution-host.md and docs/BenchmarkGuide.md, then locate the existing handler-level benchmarks they describe. Add a real-socket, end-to-end control-plane benchmark over Kestrel alongside those in-process measurements. Done means the harness measures the Kestrel and socket path rather than only the handler boundary.

Written by the indexing model from the issue text.