feat(stages): add IRequestStage and stage pipeline

[illia1f]

Last v1 checkbox before the benchmark suite. Stages are RequestFlow's answer to IPipelineBehavior<,>.

A stage wraps the handler of every request it applies to. Which requests those are comes from the stage class's own generic constraints, so no list of request types sits next to the registration.

Surface

• IRequestStage<TRequest, TResponse> and the void IRequestStage<TRequest> in Abstractions, with StageDelegate<TResponse> and StageDelegate as the continuation. Both forms share one chain, in registration order.
• AddStage(typeof(LoggingStage<,>)) and AddStage<TStage>() chain on the same configure delegate as scanning. Registration order is execution order, outermost first.
• WhereHandlerImplements<T>() narrows a stage to requests whose handler implements a contract.
• DisallowUnusedStages() turns a stage that reaches nothing into a validation problem instead of a silent no-op.

Dispatch

Chains close once, while the dispatch map is built. Dispatch walks a prebuilt array of resolved stages, still without reflection, LINQ, or locking.

Skipping next short-circuits the handler and every inner stage. Calling next again after its task completes reruns the rest of the chain (the retry shape); overlapping calls throw InvalidOperationException.

Validation

Stage problems join the single RequestFlowValidationException: interfaces and abstract classes, partially closed generics, an open generic whose contract does not use its own type parameters as the request, duplicate stage types, two declarations reaching one request as the same stage class, and unused stages when opted in.

A null task from a handler or a stage throws InvalidOperationException naming the culprit instead of surfacing as a NullReferenceException at the await.

Docs

docs/stages.md is new. README, ROADMAP, docs/exceptions.md, and docs/registration.md no longer describe stages as unbuilt. README also picks up NuGet badges and a MediatR mapping table.

Tests

Six stage test files covering closing, executor semantics, pipeline order, lifetimes, and registration. dotnet test green: 174 tests per target framework, run on net8.0, net10.0, and net462.