Docs: Command Router stream send capability

[Skymly]

Parent

#256

Related

Split from #265 after stream API lock.

• Runtime: Runtime: Command Router stream send primitives #282
• SourceGenerators: SourceGenerators: Command Router stream send capability #265

What to build

Document the stream send capability in docs/design/CommandRouter.md, and update ROADMAP / CHANGELOG / AGENTS as needed when the capability ships.

Locked decisions

• Same domain capability (not a separate Stream Request Router)
• API: IStreamCommandHandler, SendStreamAsync / TrySendStreamAsync, shared 1:1 bijection with non-stream handlers
• Reuse [RegisterCommandHandler] for generator binding (SourceGenerators SourceGenerators: Command Router stream send capability #265); no stream pipeline behaviors in this batch
• netstandard2.0 via Microsoft.Bcl.AsyncInterfaces

Acceptance criteria

• Design Doc documents stream API, failure modes, bijection with non-stream, explicit non-goals (no stream pipeline, no separate domain)
• MediatR comparison table updated (stream no longer planned only)
• ROADMAP F3 Command Router line mentions stream when Runtime + SourceGenerators have landed
• CHANGELOG Unreleased entry when user-visible API/docs land

Blocked by

Prefer after Runtime (#282) + SourceGenerators (#265) land for API-accurate Design Doc.