Set up dual reference assembly build for WinRT.Runtime

[Sergio0694]

Summary

Set up the dual reference/implementation assembly build for WinRT.Runtime. The normal build produces the full implementation WinRT.Runtime.dll, while an explicit, opt-in build (CsWinRTBuildReferenceAssembly=true) produces a lightweight reference assembly that exposes only the public API surface plus the WinRT metadata that reference projections need.

This is the follow-up to #2434 (which added the [WindowsRuntimeImplementationOnlyMember] marker and explicitly promised this change). A subsequent, final PR will wire this build into the packaging and build infrastructure so the reference assembly is actually produced and shipped.

Motivation

Reference projections compile against a reference assembly for WinRT.Runtime. That reference assembly must expose only the public API surface and the WinRT metadata attributes projections consume (e.g. [ContractVersion], [SupportedOSPlatform]), while completely omitting private implementation-detail types: ABI marshallers, native object wrappers, and the rest of the interop infrastructure.

The SDK-generated reference assembly is not suitable here: it would include those implementation-only types (since WINDOWS_RUNTIME_REFERENCE_ASSEMBLY is not defined in a normal build), leaking abstract members to downstream ProjectReference consumers and breaking reference projections. This PR therefore disables the default SDK reference assembly and introduces a dedicated reference assembly build that strips implementation details at the source level, keeping the public surface identical between the two flavors.

Changes

• src/WinRT.Runtime2/WinRT.Runtime.csproj: Disable the SDK's default ProduceReferenceAssembly for normal builds and define WINDOWS_RUNTIME_IMPLEMENTATION_ASSEMBLY. When CsWinRTBuildReferenceAssembly=true, define WINDOWS_RUNTIME_REFERENCE_ASSEMBLY, suppress the warnings that are expected only in the reference build (CS8597, IDE0005, IDE0380), and add a BeforeTargets="CoreCompile" target that removes implementation-only sources from the compilation: files opting out via #define WINDOWS_RUNTIME_IMPLEMENTATION_ONLY_FILE, all ABI\** sources, and the implementation-only Exceptions\**, NativeObjects\**, and InteropServices\** subfolders.
• Compile-time guards across src/WinRT.Runtime2/: Mark implementation-only files with #define WINDOWS_RUNTIME_IMPLEMENTATION_ONLY_FILE; split type attributes so the reference assembly carries the Windows metadata ([ContractVersion], [SupportedOSPlatform]) and the implementation assembly carries the interop/marshalling attributes ([WindowsRuntimeMetadata], [WindowsRuntimeClassName], [WindowsRuntimeReferenceType], [ABI...Marshaller]); replace method bodies with throw null; under WINDOWS_RUNTIME_REFERENCE_ASSEMBLY; and guard the using directives accordingly. This avoids sprinkling thousands of #if-s by combining file-level removal with per-member guards only where needed.
• src/WinRT.Runtime2/WindowsRuntimeObject.cs and WindowsRuntimeObject.Impl.cs: Make WindowsRuntimeObject partial and move all implementation (caching, dynamic-cast lookup, generated COM vtable handling, QueryInterface logic, activation constructors, and the exceptions helper) into a new WindowsRuntimeObject.Impl.cs, so the implementation can be excluded from the reference assembly while the public surface remains in WindowsRuntimeObject.cs.
• src/WinRT.Runtime2/NativeObjects/WindowsRuntimeInspectable.cs: Move WindowsRuntimeInspectable into the NativeObjects/ folder (content unchanged) so it is covered by the implementation-only folder removal.
• src/WinRT.Runtime2/InteropServices/Platform/ComCallData.cs and HSTRING_HEADER.cs: Apply [StructLayout(LayoutKind.Sequential)] (including the _Reserved union) to guarantee the correct native layout for interop.
• src/WinRT.Runtime2/Properties/WindowsRuntimeFeatureSwitches.cs: Suppress CS1574 (spurious XML-doc cref warning) so the build stays clean.

Validation

Verified locally that both flavors build and that the reference assembly is correctly stripped:

• Implementation build (default) and reference build (CsWinRTBuildReferenceAssembly=true) both compile cleanly.
• Implementation-only types (WindowsRuntimeDllModule, TaskAdapter, WindowsRuntimeInspectable, PointComWrappersMarshaller, IListAdapter, ABI marshallers) are present in the implementation .dll but absent from the reference .dll.
• Public-surface types (WindowsRuntimeObject, IAsyncAction, Point, AsyncStatus) are present in both, confirming the public API is unchanged.
• The reference .dll is substantially smaller than the implementation .dll (roughly 150 KB vs 800 KB).