ExecutionContext and SynchronizationContext deep dive

[BillWagner]

Contributes to #17714

1. Create executioncontext-and-synchronizationcontext.md — from "ExecutionContext vs SynchronizationContext." Covers what each context is, what "flow" means, how async/await interacts with both, why SynchronizationContext.Current doesn't flow across awaits. Most popular post (20 reactions), no existing equivalent.
2. Create synchronizationcontext-and-console-apps.md — from "Await, SynchronizationContext, and Console Apps" parts 1-3. Needs significant update: add async Task Main(), top-level statements, modern .NET runtime behavior.
3. Add both to TOC after the wrapper articles.

Summary

Describe your changes here.

Fixes #Issue_Number (if available)

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Internal previews

📄 File	🔗 Preview link
docs/navigate/advanced-programming/toc.yml	docs/navigate/advanced-programming/toc
docs/standard/asynchronous-programming-patterns/executioncontext-synchronizationcontext.md	ExecutionContext and SynchronizationContext
docs/standard/asynchronous-programming-patterns/synchronizationcontext-console-apps.md	"SynchronizationContext and console apps"

[Copilot]

Pull request overview

This PR adds two new conceptual articles that explain how ExecutionContext and SynchronizationContext relate to async/await, and how console apps behave (and how to build a single-threaded pump when you need thread affinity).

Changes:

• Add executioncontext-synchronizationcontext.md with runnable C# and Visual Basic snippets.
• Add synchronizationcontext-console-apps.md including an AsyncPump-style implementation and snippets.
• Add both new articles to the advanced programming navigation TOC.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 17 comments.

Show a summary per file

File	Description
docs/standard/asynchronous-programming-patterns/executioncontext-synchronizationcontext.md	New conceptual article explaining both contexts, flow, and await interactions.
docs/standard/asynchronous-programming-patterns/snippets/executioncontext-synchronizationcontext/csharp/Program.cs	C# runnable snippet backing the ExecutionContext/SynchronizationContext article.
docs/standard/asynchronous-programming-patterns/snippets/executioncontext-synchronizationcontext/csharp/ExecutionContextAndSyncContext.csproj	C# project for building/running the snippet.
docs/standard/asynchronous-programming-patterns/snippets/executioncontext-synchronizationcontext/vb/Program.vb	Visual Basic runnable snippet backing the article.
docs/standard/asynchronous-programming-patterns/snippets/executioncontext-synchronizationcontext/vb/ExecutionContextAndSyncContext.vbproj	Visual Basic project for building/running the snippet.
docs/standard/asynchronous-programming-patterns/synchronizationcontext-console-apps.md	New article explaining console default behavior and a custom single-threaded context/pump.
docs/standard/asynchronous-programming-patterns/snippets/synchronizationcontext-console-apps/csharp/Program.cs	C# runnable snippet implementing the custom context and pump.
docs/standard/asynchronous-programming-patterns/snippets/synchronizationcontext-console-apps/csharp/SyncContextConsoleApps.csproj	C# project for building/running the snippet.
docs/standard/asynchronous-programming-patterns/snippets/synchronizationcontext-console-apps/vb/Program.vb	Visual Basic runnable snippet implementing the custom context and pump.
docs/standard/asynchronous-programming-patterns/snippets/synchronizationcontext-console-apps/vb/SyncContextConsoleApps.vbproj	Visual Basic project for building/running the snippet.
docs/navigate/advanced-programming/toc.yml	Adds both new articles to the navigation TOC.