What's new in .NET 11 – Preview 4 docs update

[Copilot]

Updates the four "What's new in .NET 11" articles and code snippets to cover the Preview 4 release. All ms.date fields updated to 05/12/2026.

Runtime (runtime.md)

• Runtime Async: runtime libraries now compiled with runtime-async=on; covariant Task→Task<T> override support; crossgen2 inlining unlocked; DOTNET_RuntimeAsync env var removed (breaking change)
• JIT: constant-folding SequenceEqual, bounds-check elimination after empty-span guard, redundant branch/test elimination
• New section – Hardware intrinsics: F16C for Half↔float, better SIMD cost modeling, faster DotProduct/IndexOfAnyAsciiSearcher, SVE CreateWhile expansion, new SVE2 feature detection, AVX10 detection fix
• New section – ReadyToRun: Comparer<T>.Default/EqualityComparer<T>.Default specialized in R2R images (up to 20× speedup)
• WebAssembly: WebCIL V1 default for CoreCLR WASM, native re-link, JS minification, NativeAOT WASM satellite fix
• New section – >1024 CPU support

Libraries (libraries.md)

• New section – Process API expansion: Process.RunAndCaptureTextAsync, ReadAllLinesAsync, StartAndForget, ProcessStartInfo.StartDetached/KillOnParentExit, SafeProcessHandle lifecycle methods, InheritedHandles
• New section – Span-based Deflate/ZLib/GZip APIs: zero-allocation encode/decode mirroring BrotliEncoder/BrotliDecoder
• New section – Floating-point hex formatting: double/float/Half round-trip via "X" format / NumberStyles.HexFloat
• New section – UTF validation: Utf8.IndexOfInvalidSubsequence, Utf16.IsValid
• System.Text.Json: F# discriminated union support; Utf8JsonWriter.Reset with JsonWriterOptions
• New sections: FixedWindowRateLimiter RetryAfter, ConfigurationIgnoreAttribute, MemoryCache OTel metrics, UnionAttribute/IUnion scaffolding, MetadataLoadContext.GetLoadContext, Console FORCE_COLOR, TLS hardening, HTTP/2 Windows-auth downgrade
• Tar: GNU sparse 1.0 support added to existing section

SDK (sdk.md)

• dotnet watch: MAUI/mobile device selection; reliability fixes (TFM picker deadlock, Ctrl+R WebSocket, iOS Hot Reload)
• New sections: fish shell completions; dotnet reference current-directory fallback; launch-settings notice → stderr; Static Web Asset Groups; OpenTelemetry replaces AppInsights for CLI telemetry; NativeAOT dotnet CLI entry point (foundation); partial R2R for upstack tooling
• SDK tarball shrinks an additional 23.6 MB (skip crossgen for DotnetTools/-only assemblies)
• Breaking change: template engine drops netstandard2.0

Snippets

Added compilable examples for: Process.RunAndCaptureTextAsync, ZLibEncoder span API, floating-point hex, UTF validation, FixedWindowRateLimiter RetryAfter, Utf8JsonWriter.Reset.