Runtime: Command Router stream send primitives

[Skymly]

Summary

• Add Command Router stream send runtime primitives: IStreamCommandHandler<TCommand, TItem>, SendStreamAsync / TrySendStreamAsync, and builder Register for stream handlers on the shared 1:1 command map.
• Keep existing non-stream SendAsync / TrySendAsync paths unchanged; reference Microsoft.Bcl.AsyncInterfaces on netstandard2.0 for IAsyncEnumerable.
• Cover progressive yield, missing-handler, contract mismatch, and duplicate registration in unit tests.

Related Issue

Closes #282

Solution module

• Runtime (DesignPatterns/)
• Diagnostics (DesignPatterns.Diagnostics/)
• SourceGenerators (DesignPatterns.SourceGenerators/)
• Analyzers (DesignPatterns.Analyzers/ + DesignPatterns.CodeFixes/)
• DependencyInjection (DesignPatterns.Extensions.DependencyInjection/)
• Package (DesignPatterns.Package/)
• Docs / Repository (README, docs/, .github/, AGENTS.md, build/)

Note: Directory.Packages.props only adds the central Microsoft.Bcl.AsyncInterfaces version required by the Runtime PackageReference.

Type of change

• Bug fix
• Feature
• Source generator / diagnostic / CodeFix change
• Refactor (no behavior change)
• Docs / repo metadata only

Test plan

• ./build.ps1 --target Ci --configuration Release (or CiPack if packaging changed)
• Sibling samples (if API/generator behavior changed): clone DesignPatterns.Samples beside this repo and run ./build.ps1 --target Ci

Breaking changes

• None
• Yes — describe migration steps (APIs may still be pre-stable):

Checklist

• This PR touches only one solution module (see AGENTS.md)
• Commit messages are in English (no AI/agent tooling mentions in commits)
• PR description is English only — no AI/agent/Cursor tool attribution, no auto-generated summary blocks (e.g. CURSOR_SUMMARY, "Made with …")
• No version bumps, tags, releases, or NuGet publish steps included unless explicitly requested
• Public API / diagnostic / generated code changes are documented if user-visible

Documentation checklist

• Design Doc updated if API / diagnostic / implementation changed
• User-facing docs synced (separate PRs if multi-repo)
• No documentation changes needed

Docs for stream capability tracked in #283; generator binding in #265.

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Note

Medium Risk
Additive public API on core command routing; behavior mirrors existing send paths, but any custom ICommandRouter implementors must add the new members.

Overview
Adds streaming command dispatch alongside existing void/result routing: new IStreamCommandHandler<TCommand, TItem>, SendStreamAsync / TrySendStreamAsync on ICommandRouter (implemented in CommandRouter), and CommandRouterBuilder.Register for stream handlers on the same 1:1 command-type map.

Stream routing follows the same rules as SendAsync / TrySendAsync—missing handlers throw or return failure via CommandSendAttempt, and calling the wrong overload for a registered contract throws InvalidOperationException. Only one handler kind per command type (void, result, or stream); docs clarify that pipeline behaviors apply to void/result handlers only.

netstandard2.0 gains Microsoft.Bcl.AsyncInterfaces for IAsyncEnumerable. Unit tests cover progressive yield, cancellation, contract mismatch, duplicate registration, and coexistence with non-stream commands.

^{Reviewed by Cursor Bugbot for commit 0a45b75. Configure here.}