Runtime: Command Router stream send primitives

[Skymly]

Parent

#256

Related

Supersedes the Runtime portion of #265 (split after stream API lock). Sibling slices: SourceGenerators #265; Docs (filed with this split).

What to build

Ship Command Router stream send runtime primitives: progressive IAsyncEnumerable results on the same router domain, without a separate Stream Request Router. Non-stream SendAsync / TrySendAsync paths remain unchanged.

Locked decisions

• Handler: IStreamCommandHandler<in TCommand, out TItem> with IAsyncEnumerable<TItem> HandleAsync(TCommand, CancellationToken)
• Router: SendStreamAsync<TCommand, TItem> + TrySendStreamAsync<TCommand, TItem> on ICommandRouter (Try* returns CommandSendAttempt<IAsyncEnumerable<TItem>>)
• Same TCommand cannot register both non-stream and stream handler (shared 1:1 bijection map)
• Manual CommandRouterBuilder.Register<TCommand, TItem>(IStreamCommandHandler<TCommand, TItem>)
• No stream pipeline behaviors in this slice
• Attribute / generator binding is not this ticket (SourceGenerators SourceGenerators: Command Router stream send capability #265)
• netstandard2.0: reference Microsoft.Bcl.AsyncInterfaces so IAsyncEnumerable is public on both TFMs
• Core still has no MSDI/Autofac reference

Acceptance criteria

• Public IStreamCommandHandler<TCommand, TItem> + SendStreamAsync / TrySendStreamAsync on ICommandRouter
• CommandRouterBuilder can register stream handlers; duplicate command type (vs void/result/stream) throws
• Missing stream handler: SendStreamAsync throws CommandHandlerNotFoundException; TrySendStreamAsync returns Failed
• Contract mismatch (non-stream registered, stream send — and reverse) throws InvalidOperationException
• Progressive yield covered by unit tests; existing non-stream tests remain green
• XML docs; dual TFM (netstandard2.0 + net8.0)
• No source generator / Design Doc / diagnostic ID work in this PR

Blocked by

None — can start immediately.

[Skymly]

Claimed for implementation. Sibling Docs: #283. SourceGenerators remains #265 (blocked by this Runtime ticket).

[Skymly]

Implemented on feature/282-command-router-stream-runtime (commit 394fa71). Sibling Docs: #283. SourceGenerators remains #265 (blocked by this Runtime work). Nuke Ci Release passed.