bench: TCP state machine + delegate chains

[benaadams]

Problem

No benchmarks exist for TCP processing or the delegate chain overhead - the two core hot paths.

Sub-tasks

• TcpStateMachine.cs: connection setup/teardown cycles per second
• DelegateChain.cs: overhead of 1/5/10/20-deep chains, measure per-packet cost
• TcpThroughput.cs: bulk data transfer throughput (in-memory, no real I/O)
• All with [MemoryDiagnoser]

Dependencies

Depends on: #103 (TCP tests give confidence), #113 (benchmark infrastructure)
Unlocks: Kestrel benchmarks (#TBD), platform I/O benchmarks (#TBD)

Phase

Phase 2 (Platform Backends & Samples)