Centralize build properties and clean up project files

[Sergio0694]

Summary

Centralize the common MSBuild properties for all C# projects into Directory.Build.props/Directory.Build.targets, then remove the now-redundant per-project copies, fix the C# language version that was accidentally pinned to preview everywhere, and point the core projects at a single shared $(DotNetVersion).

Motivation

Properties such as LangVersion, AllowUnsafeBlocks, warnings-as-errors, analysis levels, code-style enforcement, and the AOT/NativeAOT settings were copy-pasted across roughly a dozen project files, which is noisy and easy to drift. A stray LangVersion=preview in Directory.Build.targets (imported after each project body) was also silently overriding every project back to the preview language version, even the ones that explicitly set 14.0. Centralizing these settings removes the duplication, makes it possible to try a different .NET version or language version from one place, and fixes the accidental preview language version. While validating the result, the centralization also surfaced two latent build breaks that are fixed here.

Changes

Centralized build configuration

• src/Directory.Build.props: add a Globals property group (scoped to .csproj) that defines DotNetVersion (net10.0), LangVersion=14.0, AllowUnsafeBlocks=true, Release-only TreatWarningsAsErrors/CodeAnalysisTreatWarningsAsErrors, AnalysisLevel/AnalysisLevelStyle, EnforceCodeStyleInBuild, Features=strict, GenerateDocumentationFile, and the common NoWarn entries (CS8500, NETSDK1229).
• src/Directory.Build.targets: apply the AOT/marshalling settings (IsAotCompatible for libraries, DisableRuntimeMarshalling, EmitSkipLocalsInitAttribute, TrimmerSingleWarn) to net10.0 projects, and the NativeAOT publish options (InvariantGlobalization, OptimizationPreference, StackTraceSupport, UseSystemResourceKeys, ControlFlowGuard, IlcDehydrate, IlcResilient) to net10.0 PublishAot projects.

Project file cleanup

• Removed the redundant properties listed above from WinRT.Runtime2, WinRT.Generator.Core, WinRT.Projection.Writer, WinRT.SourceGenerator2, WinRT.Sdk.Projection, WinRT.Internal, and the five build tools (cswinrtinteropgen, cswinrtimplgen, cswinrtprojectiongen, cswinrtprojectionrefgen, cswinrtwinmdgen). This also drops the leftover LangVersion=preview override in WinRT.Impl.Generator.
• Removed the now-redundant AllowUnsafeBlocks=true copies from the core projects plus UnitTest, DiagnosticTests, BuildDeterminismComponent, WinRT.Host.Shim, and the FunctionalTests Directory.Build.props. The NonWinRT test keeps its conditional AllowUnsafeBlocks=false (when TestUnsafeDisabled) since it still overrides the centralized default for that scenario.
• Pointed the core projects' TargetFramework at $(DotNetVersion) (WinRT.Internal uses $(DotNetVersion)-windows10.0.26100.1). The resolved framework is unchanged (net10.0), so output folders and existing HintPath/tool-directory references are unaffected.

Suppressions no longer necessary

• Removed project NoWarn entries that no longer fire, verified with Release --no-incremental builds (where warnings are errors): IDE0028 from cswinrtinteropgen/cswinrtimplgen/cswinrtwinmdgen/cswinrtprojectiongen, and IDE0370 from cswinrtinteropgen.
• WinRT.Interop.Generator/Helpers/SignatureGenerator.cs: IDE0370 was firing on a redundant null-forgiving operator type.FullName! (FullName is a non-nullable string); removed the ! so the suppression is genuinely unnecessary. The Writer's IDE suppressions and the projection generator's IL* (Roslyn trim/AOT) suppressions are retained because they still apply.

Build-break fixes surfaced by the centralization

• src/Directory.Build.targets: only emit [module: SkipLocalsInit] for projects that set AllowUnsafeBlocks (the C# compiler requires unsafe code for [SkipLocalsInit] or it produces CS0227). This matches the pre-centralization behavior and unbreaks non-unsafe net10.0 projects such as WinRT.Internal. (With AllowUnsafeBlocks now centralized to true, this guard continues to do the right thing for any project that opts back out.)
• src/WinRT.Internal/ProjectionInternalAttribute.cs: fix a malformed XML doc comment (a <para> closed by </remarks>, CS1570) that became an error once GenerateDocumentationFile was enabled under Release warnings-as-errors.

Validation

All core projects plus WinRT.Internal build green in Release (--no-incremental, warnings-as-errors active). WinRT.Sdk.Projection cannot be restored locally (NU1102 for Microsoft.Windows.SDK.Contracts, which CI supplies via an explicit SdkPackageVersion); this is pre-existing and unrelated to these changes. Test and sample project TargetFramework values were intentionally left untouched, so a CI Release run is worth doing to confirm nothing else relies on the previously per-project settings.