Kestrel: sample app + benchmark

[benaadams]

Problem

The Kestrel integration needs a compelling demo and performance comparison.

Sub-tasks

• Create sample/Magma.Kestrel.PlaintextApp - ASP.NET Core "plaintext" TechEmpower-style endpoint
• Benchmark: Kestrel+Magma vs Kestrel+default sockets, same workload
• Document results in BENCHMARKS.md

Dependencies

Depends on: #132, #122 (benchmark infrastructure)
Unlocks: compelling headline performance numbers for the project

Phase

Phase 3 (Advanced Integrations)