[illia1f]

Adds a BenchmarkDotNet suite on .NET 10 that compares RequestFlow with DispatchR.Mediator 2.3.1, the latest release. No timing results are published yet.

• RequestFlow.Benchmarks.DispatchR: 260 cases covering Task, void, ValueTask and ValueTask void requests, streams, sequential events, catalog size, mixed traffic, short-circuit pipelines, and direct-handler references.
• Both dispatchers run scoped, because DispatchR registers IMediator as scoped with no lifetime option.
• DispatchR has no parallel publisher and no bare void contract, so the suite has no parallel event rows, and DispatchR void handlers return Task or ValueTask of an empty struct.
• Mixed traffic sends through typed delegates on both sides, because DispatchR picks the handler from the compile-time request type. These rows are not comparable with the MediatR suite's.
• probe fails a class whose message fields do not carry its Completion parameter. The shared check also runs for the MediatR suite.
• benchmarks/README.md lists the suite, its filters, and its case counts.

Validation:

• Release build of the solution: zero warnings and errors.
• probe passed for the DispatchR comparison: 18 classes, 130 parameter sets, 260 cases.
• probe passed for the MediatR comparison after the shared check change: 15 classes, 112 parameter sets, 224 cases.