refactor: type benchmark workload choices

[0xcadams]

What

Carry benchmark suite, publisher transport choice, visitor network, and plan output format as finite types through planning and execution.

Why

The benchmark CLI already restricts these inputs, but its handwritten plan and publisher workload passed generic strings across functions. That also blurred the distinction between selecting a benchmark transport cohort and reporting an actual QUIC or TLS/TCP transport.

Changes

• Define benchmark-specific choices and a shared publisher transport choice, retaining existing enum flags and runtime checks.
• Preserve plan JSON and benchmark approval requirements while converting only at the visitor socket and reported transport boundaries.
• Pass a typed cohort choice from the separated runtime workload harness.